I'm sure it'll be permanent. The idea is probably not to leave Momo as a child while his sister is an adult. Sure he'll lose 20 years, but he'd also be able to be Shogun of Wano immediately, which works for the series as a whole.
It also supports Yamato leaving because now the argument that she needs to "take care" of Momo as "Oden" wouldn't hold nearly a much ground.
Personally, I like it thinking about it. Momo is from a different time. He deserves to rule in this time period as he should have.
But lol....does that mean Tama can now just join if she asks Shinobu to age her? There is that whole promise from Ace....(I don't think is happening lol)