Shouldn't be losing leaves this time of year. Are the stems firm? What were the roots like when you changed the soil? There is a rust which affects a lot of frangipanis, it seems to be everywhere. You could ask at your local nurseries if they have anything for it. If you wanted to, they will grow easily from cuttings, but if the stems are too far gone nothing will happen.

They don't like heaps of water, feel the stem, if soft to the touch it's history.
Try a weak solution of seasol, if that fails i would start again.
Frangipani are easy too grow from a cutting. Select a good straight piece from the mother tree, cut with a clean sharp knife and let sit for a few days on a sheet of newspaper on your patio. This allows the cut to cauterize, or 'heal over'. Select a good sized pot, fill with premium potting mix, stick it in with a weak feed of seasol, water lightly until leaves start to form (not wet) Place pot where it will get full sun for most of day. Works for me in West Oz.

how did it go? I've got a number of diff coloured frannies if you want to try again?

Mine always lost all of it's leaves in the winter and when it got warm they grew back.

Just a quick comment regarding these. I live on the Cumberland Flood Plain, in Western Sydney. Our soil is a natural hybrid of clay and concrete - I managed to break the blade of a pick trying to work it. :-) Piled trailer loads of compost on it, in the end and left the worms to do the hard graft.
As I really like Frangipani I put one in and it grew for about 3 years then fell over. Repeated the exercise a couple of times, then gave up. Each time they died, I found the trunk full of small black ants. Whether they were the cause of the plants' demise or just opportunists I don't know.

no expert here but it's possible that the soil was not draining enough. One sure way to kill a frannie is to overwater it. Maybe your soil stayed wet too long? as I said, no expert here but that's the way I've lost any frannie... they stayed wet too long.

As the stems rot they form some cavities. Ants would have taken advantage of the free accommodation. Ants don't other wise affect them.