Thanks for putting up your question on Just Answer.
1. Isn't a flexible partial more of a fixed denture?
-------- No, it is a removable denture, just made up of slightly flexible material, and without any metal hooks. It is completely removable, and in fact should be removed everyday and cleaned.
2. Meaning my gums will change shape after the extractions and ultimately won't fit? Should I be worried about a valplast partial being fitted the same day of the extractions?
---------- I wonder why your dentist has come up with this treatment plan. Why is there a need to load the ridge immediately with the valplast? The regular way to go about it would be, to extract the remaining 2 molars, wait for 6 weeks for complete healing to take place, and then make impressions. And then get the Valplast done. Your dentist's current treatment plan is more in line with anterior teeth replacement. In those cases, since aesthetics is a prime concern, immediate dentures have to be given.
I would not suggest you opt for this treatment plan , as the Valplast seated immediately will delay your healing for sure.
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Good Luck !