Upgrading packages from 2005 to 2012 : Custom components and delyavalidation woes

Question

Hey folks
I've two specific cases with upgrading SSIS 2005 to 2012
1st : most annoying
I have a custom transformation written in c# 2005. It references sql server 2005 dlls such as from 
Microsoft.SQLServer.DTSRuntimeWrap
Microsoft.SQLServer.ManagedDTS
Microsoft.SqlServer.PipelineHost
Those are stored under 
C:\Program Files\Microsoft SQL Server\90\SDK\Assemblies
The requirement is to upgrade the package to 2012 and the code as well. Since 2005 was not installed on new server , I copied the referenced dlls from old server , added them back to C# project and rebuilt the Custom components dlls then added to GAC
When I open the packages , they complain about the version
Error 2
Validation error. : The component metadata for "My_Comonents, clsid {874F7595-FB5F-40FF-96AF-FBFF8250E3EF}" could not be upgraded to the newer version of the component. The PerformUpgrade method failed.  
So I thought , I can add the dll to the toolbox and add it again. WHen I try this , I get this error
"Could not load file or assembly Microsoft.SqlServer.PipelineHost  ,version = 9.0.242.0"
Now I assume this is because I don't have 2005 components installed.
Before getting things messy , what should be the right approach :
a- Should I just leave the custom components dlls as they are and add them to GAC ? but they will still need 2005 dlls , should I install 2005 client components ?
b- I can not change the code of the c# project to use new SQL 2012 components , it's a lot of work. so what should be right approach here ?
That was the important part
2- Validation
Even for disabled tasks , like data flow task , setting delyavalidate= True still doesn't help against them being validated and raising error. removing them will help but I do not want , any workaround ?
Thanks

Answer 1

Thanks a lot Joost , appreciate the feedback
This is what I thought too.
I came to the blog you mentioned , but I wasn't sure how it should be related to my problem. Now I think I'll need to make the code changes.
Indeed , I see some declarations of *****90
public void CreateExternalMetaDataColumn(IDTSOutput90 output, int outputColumnID)
            IDTSOutputColumn90 oColumn = output.OutputColumnCollection.GetObjectByID(outputColumnID);
            IDTSExternalMetadataColumn90 eColumn = output.ExternalMetadataColumnCollection.New();
    public override DTSValidationStatus Validate() {
            IDTSVariables90 variables = null;
public override void SetOutputColumnDataTypeProperties(int outputID, int outputColumnID, Microsoft.SqlServer.Dts.Runtime.Wrapper.DataType dataType, int length, int precision, int scale, int codePage) {
            IDTSOutputCollection90 outputColl = this.ComponentMetaData.OutputCollection;
            IDTSOutput90 output = outputColl.GetObjectByID(outputID);
            IDTSOutputColumnCollection90 columnColl = output.OutputColumnCollection;
            IDTSOutputColumn90 column = columnColl.GetObjectByID(outputColumnID);
SO I should use 2012 references and start migrating this code over ? is it as easy as so ?
Thanks