1) You come up with a working (if expensive or otherwise imperfect) technical path
2) You define a half-dozen other potential (less expensive, more practical) paths
3) You announce the decision
4) You complete due diligence, and take the best path
There are other ways as well. Announcements and plans aren't binding; they occasionally change. You can make an announcement when you're 98% confident you'll do something. You can pivot if it doesn't work out. There are places this doesn't work (e.g. customer promises), but on something like an internal migration, this is a-okay.