Question regarding Japanese Phone Number

Hi all! I've been doing some searches about this topic here and there and have found people with similar questions but no concrete answers so I'm making this post for myself.

I'm currently an exchange student, but I'm stayed to leave Japan around August 20th. I currently have a Japanese phone number with LINEMO and I've used it to register on a lot of different Japanese websites that require a Japanese phone number like Yahoo Auctions and Mercari and Ticket Pia and the like.

I was wondering when I leave the country, would it be recommended to cancel my LINEMO contract? My main concern is the SNS verification required for some of the websites I've used my Japanese phone number to sign up with. If I return to Japan, how easy would it be to change the phone number on those accounts if I no longer have access to my old Japanese phone number anymore?

For reference, I'm not sure when the next time I'll be in Japan again, as I have to return to my home country and finish my studies.

Thanks!