OK, so I'm going to throw a monkey wrench in this. Why are you switching from using your strike ball to using your spare ball if you were nearly 100% on left sided spares? For left sided spares I am NOT counting the bucket. Because leaving the bucket is an obvious symptom of a bad strike shot. Therefore, you should not totally revamp your bowling system for a bad shot. So I am thinking you are having trouble with the single pin 7 or 4 pin leave.
Are you expecting to bowl on sport, flat and PBA conditions on a regular basis? If so then you are going to have put up with the learning curve involved. And I support this.
If not, then I don't see any reason to change. If you think that you might bowl a sport, flat or PBA condition maybe a couple times a year, then IMO, its still not worth switching.
Its a little odd, for right handers to have trouble with left side spares (I'm not counting the bucket). But I struggled with the 7 pin more so then the 10 pin. When I started seeing more synthetic lanes, the 7 pin and 4 pin got much harder to convert. The lanes just didn't hook like I was used to. And I did try shooting the 7 with my spare ball to no avail. But I did practice and concentrated and put thought into what exactly I had to do to make those "simple" spares dependable. As usual it all came down to practice and repetition. So if you are determined to use your plastic ball, then you are going to have to put in time at the center, just shooting 7 and 4 pins.
I'm a pretty good spare shooter and when confronted with such patterns I still use my strike ball and I still have good results converting my spares. I just adjust my existing system for the condition. But I have a spare system (on a house shot) that is pretty darn solid. So maybe, I'm suggesting, that your spare system on a house shot with your strike ball, needs to be more secure, before implementing a secondary spare system with a plastic ball. But having both in place would be a good advantage for when you see conditions that are just plain messed up. But I would still fall back on your strength. If that's a strike ball then so be it, if you end up (with practice) to be a plastic ball then that's fine too.