We went through the same procedure with one of our customers back in 2008. They had been running the SharePoint Portal Server 2003 (with RtWebParts) since 2005 I believe.
We did the upgrade with no big showstoppers, just following basic guidelines from Microsoft. A quick google search gave me this page: http://technet.microsoft.com/en-us/library/cc303420.aspx
The RtWebParts were also upgraded (to version 2.1) without major problems. (There was however an issue when upgrading from 2.1 to 2.2 on the server which USED to run SPS2003, and is NOW running MOSS2007: The RtWebParts installation tried to target the IIS Web App. which USED to be the default web app, and now doesn't exist. Pretty straightforward fix for the techsupport guys though. Just go into RegEdit and change the 'default webapp' setting to its correct value)
The situation today with this customer: We're planning on virtualizing the one back-end server and the two front-end servers of this installation - and also take the opportunity to install MOSS2007 from scratch, as there are a few references to the old sharepoint installation which still cause smaller problems for us!
So my advice to you: Why go the extra route around the SPS2003 when you can go directly to MOSS2007?
Thanks for the information. I also found that Microsoft article. I am glad to hear that the migration from the OSI side is fairly simple. At that time, the client perferred using SPS because of the cost and not having to alert the internal IT group. After discussing the options with him, he is going to see if we can use their current MOSS install and create a site for the PI system. So, might not have to worry about the upgrade.
I thought I would throw in my two cents. I did an upgrade of a SPS2003 setup with RtWebParts backing 2008. Our main reason was EWA (Excel Web Access/Excel Web Services). The upgrade I must say was pretty painless; no major showstoppers. This included the RTWebparts upgrade (v2.1 if memory serves me correctly) which went smoothly. One issue we did have was a third party webpart did not like MOSS; the solution was simple, stop using it. Microsoft changed the version ASP.NET for MOSS it actually use the same version as the main .NET framework; whereas SPS2003 used a specialised version of ASP.NET. MS also changed the package descriptor format in MOSS2007 however MOSS still reads the older dwp packages. The MS documentation on the upgrade is fairly comprehensive so I strongly suggest reading it (http://technet.microsoft.com/en-za/office/sharepointserver/bb421259(en-us).aspx). I would suggest that you run the upgrade advisor first to check for major incompatibilities. I would also advice avoiding 3rd party webparts that don't have an upgrade path to MOSS (i.e. if they don't specify support for MOSS2007 by now they probably will bit you).
One small issue I feel I should mention is that after the upgrade the site structure was a bit messy from a technical perspective (nothing the users would see). The site and db had all sorts of references to site_pairs; the old SPS2003 databases. A bit messy but not an serious issue; we've been running the site for 2 years like this. Another small issue was the backups; after upgrading MOSS the backups broke and we didn’t notice it for awhile, interestingly this has an impact on performance and MOSS appears to only cleanup the transaction logs in the database when it performs a backup (the performance was terrible when we had a 20-30GB log file).
On slightly different topic. What is the rationale for going SPS2003 now when MOSS2007 is stable and MOSS2010 is imminent? One possible option would be to go the WSS3.0 route which is "free" with Windows 2003/2008 server (please note the quotes as you still need CALs). Depending on what features you want you will probably get 80% from WSS3.0. You will be missing the following features (off the top of my head so this is not an exact list): EWA/EWS, BDC and I think SSRS integration requires MOSS (could be wrong); you can check the features list on MS’s website. Bear in mind the differences between MOSS std and MOSS enterprise (most of the features I’ve list above are in MOSS enterprise).
I appreciate the feedback on your lessons learned. I will definitely heed your words about custom webparts if the customer chooses to go down the SPS path. I will also look into WSS3.0 if we can't use MOSS. In my searching I noticed a thread from someone who had come back to their migration system and spent the time building a new MOSS installation from scratch and porting from one MOSS to another in order to remove the old SPS references. I guess that might be a solution for you if you find time.
Thanks and have a great day,