I know we are all stunned and saddened with the cancelation of the Peacock's SAVED BY THE BELL, as it is almost certainly the end of the line of the franchise.
I know we can look back and see where things were done right, done wrong, could have done better. When a franchise is as successful as it was, I think they did just fine. Still, I know a lot of us have ideas and or dreams of what could have been to make it better.
IF I COULD DO ANYTHING DIFFERENT........
If it was clear that in 1992 that Elizabeth Berkley and Tiffani Amber Thiesen wanted off the show for the reasons given, which is totally understandable. I would have started thinking about the next generation and even though the New Class lasted another 5 or 6 years I think, it was clear they never really "caught on".
My answer to this would be, of course, you look at the number of episodes that you have Jessie/Kelly for. Backload them as much as you can, to have them do what they did, and have them go "to the semester at sea" program that they ended up doing to justify them being gone. But by that, you could have them gone in early part of that last season.
I would have then, at that time introduced a wave of characters both at Bayside and at Bayside JR High. You could tie up a loose end of the show, where Zack/Slater/others go to the JR High, meet up with Miss Bliss one more time and take to mentoring a few 8th graders that are going to be coming, as well as some underclassmen at Bayside.
Use a bunch. See what tests well, what the audience reacts to, and then, when Zack and the gang graduated, you have the audience already invested in the new cast of characters. Maybe it would have done better there.