My guess is the commanded speed would remain unchanged, flaps do not change speeds on the 744 ( they do on the 748).
On an RNAV approach you would have the MCP speed window open, since after passing the approach point ( which is the point on position R6 on the dep/arr page when you select the approach) system does allow speed intervene with the aircraft remaining in VNAV path.
I might be wrong, I didn't look it up, just from memory.