In Finland getting your drysuit zipper is expensive. In 1998 there wasn't any diveshop that would replace the zipper for a neoprene drysuit. Nearest one was in Stockholm, and the costs would've been 3000FIM ($600USD) plus shipping, so we thought that replacing the zipper cannot be rocket science.
First we checked if we could get a zipper in Finland. That wouldn't be a problem: Ursuk would sell one and deliver it by mail or via the nearest dealer. Their price would be reasonable as well, only 700FIM ($140USD).
Then we researched the possible glues and techniques to use for this kind of work. Scubapro gave us information about the glue and suggested that we would cut the zipper off from the suit near the edge of the neoprene. The new zipper would be glued on top of the remainder of the old zipper. He also told us one suitable glue, which was Dunlop S2000.
Now we have everything we need and we can begin the work.
Taking away the old zipper was trivial. You can use either a very sharp knife (like carpet knife) or scissors. Be careful not to cut the neoprene.
We turned the drysuit inside out so that the gluing would be easier. We also placed a plank of wood inside so that you wouldn't glue the suit shut and it'd be easy to press the surfaces together tight.
Some glues work better if you rub the surfaces with sand paper to make them rougher. So was the case with the glue we used as well.
Next we had to clean the surfaces. For this we used toluene. It is important to get all the grease that you might have got on the surfaces away. You need to be careful with the solvent since they caught fire easily and many of them are carcinogenic.
When we were ready to start gluing we had to mix the glue: Dunlop S2000 consists of two components which have to be mixed prior usage. The mixture will be usable for 6 hours.
Then we opened the zipper. After that we applied the glue to the surfaces. Also the solvents in the glues can be dangerous, so you have to take care of ventilation. We took the suit and the zipper out to the balcony, so the ventilation was sufficient. We applied the glue to both of the surfaces and then we let the glue to dry for a while, as instructed in the instructions that followed the glue.After the designated period we pressed the surfaces heavily together. Older neoprene suits have wrinkles which can be sharp, and you'll need to be extra careful with those. We worked like this:
When the glue was dry (after 24hrs) we did a visual inspection on the work we had done. Although it seemed that not every wrinkle would hold water we decided to test the suit in the pool.
The pool test proved our suspicions correct: the water was seeping in. Because we couldn't fix the gluing itself we decided to fill the holes. After the suit was thoroughly dried we applied LiquiSole on the suspected holes. Now it was time for another pool test.
The suit held water outside! We used about one evening for the entire process, plus 800FIM ($150USD), so money savings were substantial. We have since used the suit a few times in the open water and we have had no problems with it.
Probably you could use the same technique with normal trilaminate suits as well. You could probably remove the old zipper as well, if you want to. According to what I've heard you can apply heat with for example hair dryer, or you can use a suitable solvent. Probably the gluing process is easier because the suit is less likely to have wrinkles.