How do I force lightroom to (re)load an XMP sidecar file?

Question

Good morning from Boston!
I am new to Lightroom and just finished importing my Aperture library into Lightroom 5 on a Mac. For that, I followed the instructions on Adobe's landing page and Lightroom solutions. For those not familiar with the instructions, you essentially export all your originals with an XMP sidecar file and then export all the adjusted/edited files as 16-bit TIFFs. Then you add them all back into Lightroom and get everything back into Catalogs, Labels, Ratings, and such by using keywords that were added while in Aperture. It took forever, but worked pretty well overall. I had about <1% fewer images in LR5 than in Aperture, which were likely suspected duplicates. So far, so good.
Here's is my problem. My keywords didn't always transfer over (the Keywording panel is empty). I did search forums and plug-ins, but seem to only find issues related to EXIF and GPS data. I also used "Metadata:Read Metadata from File" and "Metadata:Update DNG Preview & Metadata," which didn't do anything. I am suspecting that "Read from File" means "read from the image file itself but not from the sidecar file." Take file IMG_5888 as an example. This particular one was taken as RAW (.CR2) + JPEG on a Canon Rebel XS, but I am having the same issue with RAW only on my Canon EOS 6D. Same goes for JPG + XMP. Puzzling thing is that other files loaded just fine.
In the directory/folder where the file is, it looks like this:
IMG_5887.CR2
IMG_5887.JPG
IMG_5887.XMP
IMG_5887.dng
IMG_5888-2.JPG
IMG_5888.CR2
IMG_5888.JPG
IMG_5888.XMP
IMG_5888.dng
IMG_5889-2.JPG
IMG_5889.CR2
IMG_5889.JPG
IMG_5889.XMP
IMG_5889.dng
The .dng were generated by Llightroom. Now, the sidecar files typically have something like so in them (I cut out some keywords for clarity—you get the idea):
<rdf:Description rdf:about='' xmlns:photoshop='http://ns.adobe.com/photoshop/1.0/'>
</rdf:Description>
<rdf:Description rdf:about='' xmlns:dc='http://purl.org/dc/elements/1.1/'>
  <dc:subject><rdf:Bag>
  <rdf:li>Black Head Trail</rdf:li>
  <rdf:li>Cliffs</rdf:li>
  <rdf:li>Maine</rdf:li>
  <rdf:li>Monhegan</rdf:li>
  <rdf:li>RAW</rdf:li>
  <rdf:li>Sunrises</rdf:li>
  <rdf:li>United States</rdf:li>
  <rdf:li>Waves</rdf:li>
  </rdf:Bag></dc:subject>
</rdf:Description>
So, the keywords were exported from Aperture, but for some reason, Lightroom isn't always loading them from the XMP file.
On a related note, the keyword hierarchy that was exported as text file from Aperture and imported into LR5 worked well. Everything appears to be there and in its place.
Any ideas for a Lightroom rookie as to how I can get LR5 to read those XMPs? Over 20,000 didn't tag, so I would rather not do this by hand!
Many thanks in advance,
Paul

Answer 1

John, what a pleasure. Well, here is the update. First, copying over 300GB of pictures to a drive and then backing up said drive takes a bloody long time. But here is what happened, for others who might run into the same issue.
I first tried a small set of files to narrow down whether the issue was the Aperture export or the Lightroom import.
Conclusion: the "issue" is mostly on the Lightroom side.
What I found: Although Aperture will export an XMP sidecar for a JPEG, Lightroom will not read it, ignoring all keywords. I exported a set of JPEGs only and then opened the XMPs with an editor (well, the UNIX cat command, to be exact). The information was there, keywords and all. However, LR appeared to ignore XMP for JPEG. That said, if you have a RAW+JPEG in Aperture, the RAW XMP sidecar will overwrite the JPEG one, effectively leaving you with only the keywords for the RAW version. That shouldn't be an issue for most, but just in case you have different keywords for your RAW and JPEG images, be aware of this.
What I did:
Back up all the images and catalogs/libraries from Aperture and LR.
Make sure #1 worked out.
Make sure #1 worked out (not a typo; you really need a backup, as you will find out below).
Delete the old JPEG files that were uploaded into LR. I had them in a From Aperture folder, as instructed by John's article and Adobe's landing page (referenced in John's article), so I deleted all JPEGs in that folder. My Aperture database was not affected and in fact resides on a different drive.
Run find . -iname "*.JPEG" -delete   [if you do not know how 'find' works, ask someone, as this can potentially delete every single file accessible to your computer!]
Run find . -iname "*.JPG" -delete    [ditto! Don't tell me I didn't warn you!]This is why you need to backup everything. I was deleting copies/exports of the originals in Aperture (again, read John's article for the difference) and still had the originals in Aperture, on a different drive.
[I didn't worry about the unmatched/orphaned XMP files for the JPEGs only; they are tiny and don't hurt.]
In LR, select 'Library:Find All Missing Photos.' If asked, remove them from the Catalog only.
Create a Smart Album in Aperture and include only 'Filetype is JPG.'
Select all images in that album.
Export those original images per Adobe's landing instructions, but for this set of images, select 'Include IPTC' instead of creating a sidecar file.
When Aperture is done, open LR and Add the whole 'From Aperture' folder back to LR (make sure 'Ignore suspected duplicates' is selected.
John, as you rightfully pointed out, this is tricky. If my experience is any indicator, I would suggest migrating in 3 steps:
Export all RAW originals with IPTC in XMP sidecar files.
Export all non-RAW originals (which I would guess are mostly JPEGs) with IPTC included. (not a sidecar)
Export all versions as 16-bit TIFF.
If #2 is added, then the process is actually most straightforward. Steps 1–5 in the first block above were to clean up the mess that I had created and should not happen otherwise (famous last word...)
If the above is unclear, please let me know and I will expand on it. If I had the issue, I suspect others will as well.
Thank you for your help and suggestions. I am officially migrated to LR and up and running!
Cheers from Boston,
Paul