Jelinda: I'd remove it from the current container to take a look at the roots and see if it's possibly root-bound. I'd think it would be fine to repot in fresh potting medium as long as you keep it in a bright, warm location indoors.
I've always had trouble keeping Maranta's and Calathea's alive for long but I have two Calathea plants that began looking ratty and I sat the pots beneath shrubbery in deep shade and they began to look really great! I need to check on them because we did have a couple of cold spells recently.
