You do not need to change your BOINC credit into GRC, BOINC credit is a recognition of how much work you have done for BOINC projects. Your Gridcoin wallet (or in your case the pool) will handle converting your BOINC credit into GRC if you have everything setup.
Gridcoin works on RAC (Recent Average Credit) to represent how much BOINC work you have done in recent times, so its important to keep processing BOINC work on a regular schedule, this can be 24/7/365 or you can do less, but for higher payout more is better. Your RAC will continue to increase for around 4-6 weeks if you keep a regular BOINC processing schedule.

The pool will payout on its own schedule, you need to check with them when your payment is due, I think its ususally 2 weeks, but I never used a pool.