iPhone 7 to iPhone 13 storage management for iCloud +

I have just got a new phone and it is a massive upgrade from my old iPhone 7 that I have had since 2018 from my niece. I have been getting her older phone for ages and I have never had the reason for buying one but I got a iPhone 13 I just couldn’t afford the pro but it’s a good lesson for the future. I got the same 128gb and it’s still just under half full in my phone but iCloud + is almost at the limit. So I would like to know if the storage is still usable on the old iPhone so that I could make the new one storing less?

thanks

[Re-Titled By Moderator]