outdoorxplorer,
The only problem I encountered was when I wanted to switch back and forth between 3G and GPRS (to save battery life).

Once 3G is activated on a lower-capacity SIM card, it could lose the ability to revert to GPRS. A call to CS confirmed this as a known issue and I was offered two options:
(1) pay Maxis $25 to upgrade to a 3G SIM card which would give me the freedom to switch between 3G or GPRS
(2) deactivate the 3G service

This problem might not affect ALL non-3G SIM cards (depends on your luck) and in your case-after activating 3G-you might not want to use GPRS ever again. So, nothing to worry about.