First of all, it was important for me to test both options as scheduled jobs rather than Ad-hoc. Mainly because scheduled fulls behave differently (chain is NOT modified). And you cant launch a Synch full manually.
So the plan was to set the job with Active full, record performance, then change to Synch Full, record performance.
1) Started off of a clean slate, no chains. Retention set to 5 recovery points.
2) First job set to run Active Full on Fri at 12:00. The job ran successfully, got an Active full. Recorded performance values.
3) I added 2 incrementals into the pot (unchecked the active full and ran the scheduled job 2 times). Success.
4) Next job set to run Synth Full on Fri at 13:00. The job ran successfully. But to my surprise another incremental was created!
5) Rescheduled to run in 2 minutes, just to confirm the settings. Again, incremental created.
6) Changed the Synth full back to Active full, scheduled for 13:30. Again, incremental created.
7) At the same time I am noticing that my chain is at 6 recovery points now, despite my retention policy set to 5.

9) I uncheck the full backup, effectively scheduling just a normal incremental. Incremental is created and the chain is trimmed to 5 recovery points.
10) I scheduled a Synth Full for Sat 12:01AM. Synth full ran successfully, thus proving my point below.
Assumption: If your job has already run a full today, you won't be getting any more fulls today, even if you change the job settings. The job will run but you'll be getting incrementals instead. And because the scheduled job is configured with a full, your chain will not trim.