Hi Jim, hope you are well.
Can't really convert an existing copy from Synchronous to Selective, however here are a few options I'm thinking of for you.
1. Since you only want Fulls, drop the retention on the existing Synchronous copy to 1 day 1 cycle, and under the Extended Retention rules set retention for "All Fulls" this way the Inc's will age off quickly and only Fulls will remain.
2. Create a new Selective copy, and choose the Synchronous copy as the source (under Copy Policy) tab, and copy over the jobs you would like (i.e Weekly Fulls) based on the criteria you need on this Selective, and once the copy is done, you can remove the older Synchronous copy altogether.