simplest reason for not allowing import after you start is that could change some of your entries.
since you can not e-file 2020 print out schedules D and the 8949s from what you've done. then in the new return just enter summary totals for each type - 6 lines maximum needed. then substitute the printouts for those dummy pages. thus no need to re-enter all that work and you can use this method for most any other form or schedule that is complete
