It's just be more convoluted and screw up continuity more. Keep him a nephew - maybe change his middle name in next reboot if you're keeping Wally's dad Rudolph, but even that's unnecessary.
Also, I loathe age-regression - if accompanied by memory-regression, it's destroying the person that used to exist. Touched on
here, though that's Bat-fam specific.
Thirdly - why would he need to go to another planet to relearn Flash abilities? Every first Flash on every world (and numerous other speedsters) learned just fine with no one to teach them. It doesn't make logical sense.
Then, of course, he'd want to, you know, return home. So he'd be removed from the main universe. Or still here and stuck away from loved ones.