I have not tried it, but my suspicion is that it will work. Based on the GDB compatibility matrix for 10.2, it appears that client-GDB connectivity is supported for several previous releases. Assuming that same logic still holds true for the 10.2.1 release, you should be ok. That being said, I think 10.2.1 is considered to be similar to a service pack rather than a "new version". Esri is not issuing SPs now in the same fashion as before; instead they are issuing dot releases. So I think a 10.2.1 client should be fine with a 10.2 GDB. I will say, however, that upgrading your GDB from 10.2 to 10.2.1 shouldn't be painful. The leap from 9.3.x is much, much worse than this path. The 10.2.1 Upgrade Geodatabase GP tool should not encounter any new issues that the 10.2 tool did not already. Moreover, it really is best practice to keep everything in the same in terms of release/build numbers. Can you try upgrading in a test database first and use the alter field tool to see how it behaves?