A jet tub is one that has powerful air jets which bubble the water, while a soaker tub is one that simply holds and contains warm water for a long soak. If you want to just have a long soak, there are ways to convert the jet tub to a soaking tub. Reinstalling a new tub is one way and probably the most effective; however there are more convenient solutions that are less expensive and time-consuming.
For a simple solution, remove the jet tub heads and resurface the tub with a new bathtub liner or professional resurfacing product. These liners and resurfacing solutions will cover the existing tub in a waterproof layer. Similar to a shower liner, they are custom molded and fit to your tub. This solution works with most jet tubs if they have fitted jet heads. Contact your professional tub liner installation pro to inquire about their resurfacing options and whether this solution will work for your tub.
Another option is to reinstall the bathtub, removing the old jet tub and placing a new one in its place. This process requires a great deal of work. Rip out the old tub by first removing the jet heads and then the plaster and tiling surrounding the tub (in addition to the tub surround). Remove all drains and faucets if they are installed on top of the tub shell liner. Now remove the tub shell by cutting the caulking and prying it up from the tub basin. Drop the new tub shell into place.
Soaker Tub Installation
To install a soaker tub, fit it to your tub and shower area. Secure the tub shell in place. Apply a bead of caulking around the tub along the seam and replace the surrounding tile or bath board. Next fit the faucet and drain to the tub.
It is not recommended that you try to resurface the tub yourself with fiberglass or another compound after removing the jet heads. This will result in leaks and will not provide a satisfactory result, as the process is quite complex and is best done by professionals.