I've had two bonsai trees for around 15 years now. I moved to Japan for 6 years and left the trees with my mom to water them -- back in the U.S. now I'm looking at the trees and thinking they probably need to be repotted in larger pots. I took a class on bonsai trees about 15 years ago when I got them initially, and remember the instructors strongly recommended only repotting in the Spring -- reading around online it appears the same advice is common online as well.

The Spring-only repotting advice makes sense for bonsai trees that spend some time exposed to the outdoors, but I'm wondering if it also applies to indoor-only bonsai trees. I'm around Ann Arbor, MI where we have cold winters, but usually keep it warm enough indoors for our plants to be comfortable.

Attached photos of both trees - one seems quite happy, but one seems increasingly unhappy, potentially due to needing a larger pot with fresh nutrition in the soil. Trying to figure out if the latter one needs a new pot urgently enough to attempt it now, or if I should wait until Spring.

Appreciate any suggestions.