Is it possible to amend a product/device claim to a method claim after receiving an office action? Where is this discussed in MPEP?
Yes, claim amendment is allowed during an Office Action as long as the amended claim language fits within the original disclosure and background.
The information you are looking for is in MPEP Chapter 0700 (Section 714) 37 C.F.R. 1.121 Manner of making amendments in application, under item (c) Claims.