...to avoid the problem skoormit pointed out for player 1, his fleet 1 should have two ships.
That way, when he does want to scrap that design, all P1 has to do is to xfer both those ships from F1 into a new fleet Fxx... and split another fleet (in space) Fyy into F1.
Or just have the last player keep his 10th base slot empty for those couple of years, if he only has a blueprint in P10SB10.