For CS calls nothing will happen as all are in GMSK. By the way, BCCH TRX has to support as well GMSK since BCCH TS and SDCCH are only in GMSK modulations.
In your case, I understand you are talknig about voice call… your TCH will be GMSK from the beginning.
8-PSK average power is lower due to the modulation itself (peak power remains the same). This has no impact on voice coverage, and hardly no impact on EDGE performances.