What is the term for a deployment designed to make a future cutover seamless?
... by making a minor change in the v1 front half, this breakage can be avoided.


The above excerpt of this queston reminds me of something similar from around the late 90s, say 1996-1999 (when zillions of ITers were preparing for Y2K ...). In those days, new software releases of huge software vendors would come with release notes that included things like either of these:


"this release is to make this software Y2K tolerant " (= allows the software to contnue to work as of Jan 1, 2000, at least for a while, though it would keep showing dates with 2 digits for the year, like 00, and not yet 4 digits, like 2000).
"this release is to make this software Y2K ready " (= allows the software to continue to work as of Jan 1, 2000, without any (known) limitation because of the change of century from 19 to 20).


Y2K tolerant softwares would typically also include some sort of magical cut-off value, e.g.: years from 70 (when even MS DOS did not exist ...) to 99 were assumed to be from 1970 to 1999, while anything from 00 to 69 were assumed to be from 2000 to 2069. Such assumption was also why such softwares were required to be superseeded by some Y2K compliant release later on (in which those assumptions were obsoleted).

With the above in mind, the term you are asking for could be something like a "V2 tolerant release", which at some point in the future would then be followed by a "V2 ready release" (= your actual V2 release).