Upgrade path from Access 2003/SQL 2003 ADP to cloud, Azure, etc.

Question

My company uses a massive client database to track all interactions with our clients -- phone log, billing info, authorized users, associates, products and services they subscribe to, etc. -- as well as separate records for leads and clients. There are
over 25,000 clients and leads in the database and each client or lead has at least 30 selectable options plus numerous phone/interaction log entries.
The database is in MS-SQL 2003 Server with an Access 2003 ADP front-end running on Windows Server 2003, accessible from over 30 user accounts on the WS2003 box and accessed either over RDP or in-house XP clients.
Therein lies my problem. I did not build this monster, but am now the servers admin for this company and so it has become "my problem". Windows XP and Office 2003 are already past EOL and Windows Server 2003 will be EOL in July 2015.  However,
Access 2013 does not support the ADP file format and so I cannot open the ADP on a newer computer. 
Rewriting the database and its records -- estimating there are over 50,000,000 records in this database -- is prohibitive. My employer would rather use the WS2003 servers to perpetuity than rewrite this beast. However, knowing what I do about the security risks
and the confidential client data stored in it, I am arguing that we must eliminate all of our XP/2003 servers ASAP or we will be the next Target or Home Depot. 
What options do we have? Cost is a factor but not a deal-breaker since we have to do something. 
Ideally, I would like to be able to migrate the MS-SQL 2003 back end to Azure SQL; then hire a programmer to rewrite the .ADP Access 2003 file into either a .NET application or a MS Access 2013 front end.  However, we cannot afford much downtime for this
-- as our 30+ staff are routinely and simultaneously entering and transcribing phone calls throughout the work day -- and whatever we do still needs to be able to be simultaneously accessed by multiple staff members.
I should add that I am not going to be programming this myself; I am the department head and supervisor of this project, but will be putting together the job description and work order for the developer we hire or contract with to do this job. So what I need
from the community is a recommendation on how we should go about this:  Which services, which software, and how should we implement it?
Thank you in advance.

Answer 1

It seems to me that the most critical component is the ADP file. Some later version of Access must be able to open it. Or?
Then again, this is not an Access forum.
From the database-side of things, it is not inconceivable that the database could survive the ordeal even if you replace the Access client with a .Net client. Of course, if you are on SQL 2000 (as Kalman says, there is no SQL 2003), their is a fair chance
that you are using some deprecated features that have been removed. Still, that would not be a showstopper.
As for downtime, there is no reason to have any downtime while the new application is being developed, but you may need some downtime when you do the migration itself. Typically, that would be on a weekend. And of course, it needs planning. If a downtime
during a weekend is not acceptable, migration could be done incrementally, but it would take more time to develop and test.
I think your first step is to bring in a contractor with experience of application development, both client-side and database-side who can study the case more carefully and from this make recommendations. Obviously, that would be a senior consultant.
Erland Sommarskog, SQL Server MVP, [email protected]