old style was threaded 3/4-16 and took a simple collet and nut. decent, but not anywhere near the availability of collets and sizing. also, it wasn't as accurate for runout, nor did it have the clamping (compression) range of a decent er collet. you may never notice depending on what you are using it for.
you have a couple of options;
1) adapters are made to go from 3/4-16 to er16. looks kind of like a toolholder. you put it on the thread you have in place of the nut, you get er16. advantages; cheap and you get all the er16 collet sizes. disadvantages; you still lose out a bit on runout due to the adapter adding some, you also lose a bit of z due to its length.
2) replace the whole spindle, they are only like $100. advantages; lots. disadvantages; costs $100.
3)use what you have. they aren't bad unless you are using doing things -very- intolerant of runout, or you want to use metric and odd sized tooling taig does not have collets for.
the 3/4-16 spindle is still used on the taig lathes, which makes sense as thats what a lot of the lathe tooling is threaded for. as such, i recycled my old-style mill spindle and converted it to a rotary axis/indexer as you can get a chuck from taig (or sherline) to fit right on that thread, and the dovetail plate fits right on the mill table.
so, if you wind up with one extra, auction it off as a current lathe replacement part, or convert it into an awesome rotary table for turning with the addition of a chuck, stepper motor, and a timing belt and pulley.