I've done a lot of reading on wheel building, there is a lot info out there but finding good useful info requires lots of reading, note taking and comparison.
My conclusion was that when changing a rim hoop from stock, you may be ok with stock spokes, you may not. Some rims are made of thicker material than others, especially when you consider aluminum rims.
The best way to be sure is to measure your new rim's ERD effective rim diameter, then measure specific dimensions on your hub, plug this all into a huge equation which yields the inner and outer spoke lengths you need. If the difference is small it may be a non issue and you can use your stock length spokes. If they're too long you may be able to get away with trimming them just a little.
I'll try to find and link the web page that made it all make sense for me, it had useful explanations and automated calculators for those like me who are allergic to long equations.
