Robert

Sounds like all you need to do is put your serissa into a larger pot. You can call this a transplant, but it's not really......just take it out of its current pot, not disturbing the soil at all, and then place the whole thing into a larger pot, having put some new soil in the bottom of the pot first, then sitting the root ball on top, and placing new soil around the old. This should relieve any compaction or drainage problems for the present.......then, in the fall or spring you can re-pot in the regular way; replacing most of the old soil and trimming some roots, etc.
It's a simple, but immediate way to get your tree happier without disturbing it at all.
Robert