The time line has yet to be determined. Reports appeared in The Attleboro Sun and Universal HUB about this. MassDOT has apparently put the work on hold as some folks who had not heard about the change as it went through the planning process including bids awarded in December 2015 are now raising issues.
It is fairly common as the Federal Highway system is encouraging a conversion to exit numbers aligned with the milage as opposed to the current simple numerical sequence. So Exit 16 on i495, Franklin would become Exit 41 and Exit 17 would become Exit 43.
The full listing of exit number changes on the major highways can be found here
The i495 list is found here