My first train of thought is that you are very much correct in your processes. I would also say for you to look at the View 5.1 upgrade guide. This includes some steps and warnings about how to upgrade the composer. These same "pre" steps would need to be applied before you disconnect\stop the composer. The largest aspect being disabling refreshes, recomposes, and previsioning. During my test upgrade I missed changing the pool setting that made the machines refresh on disconnect..lol 
. Once the composer is no longer going to be utilized you should be in the clear as far as live environment interruptions.
I would then detach the databases from the SQL server and make a copy of the files for the VCDB and ViewCompDB (depending on how they were named). Then just move those files to the new server and attach them to the new SQL install. After that in theory it should just be a few software install steps and ODBC setups away from having it moved. I would also be sure to reattach the original databases so that you can return to that setup if anything goes wrong.
Once that is done I would just name the new server with the same name and IP of your old one as you said. I have not done this type of "upgrade" but based on what experience I have had with cleaning up our SQL install\file placement and upgrading to View 5.1 I would expect that this process would work out. The biggest thing I would stress is to be sure you can fall back to your old setup if anything goes a stray. I would also possible watchout for needing to re-add the Composer or vCenter to View Manager when done.
Anyway just my rambling thoughts on this topic. Please let me know if you find any of it useful or just way off, and good luck with your upgrade. 