As long as the plants don't dry out too fast they should be fine. I have several plants that are probably a little under potted and the surface of their potting mix is almost all roots. The plants grow well as long as they are not allowed to dry out.
If your plants that are in small pots are actively growing potting them up may be just what they need. Some plants will just keep on growing after potting up while others appear to stop and focus on root growth for a while.

Mike

I'd move them up to a bigger pot myself. I automatically repot anything I get anyway (except those hugely full EA plants that are next to impossible to repot) because I want them in my own mix, because I like to examine the roots for any problems, and because usually they ARE potbound and I prefer to give mine more room. Though, as Mike says, sometimes they go semi-dormant when you repot, it almost always results in super vigorous growth once they're settled in. I sometimes let mine get potbound due to timing (prefer not to repot when blooming) or I'm in a busy time of my year when I don't have a lot of extra time, but I try never to let them stay potbound for too long.

I vote for repotting.

Denise in Omaha

I personally would top it off with your soil mix.

Gabi