Actually the term reglazing is not what it may seem.
Many companies do what they call reglazing.
Some just paint over the tub and some are better than others at it.
I have seen painted tubs last 6 months to 10 years but most come loose or start to peal or flake within 2 years and then to have it redone it must be stripped and it is a difficult process.
Most real reglazers do the following:
Clean the tub
sand down the stained or bad areas
fill in or repair any cracks
glaze with a chemical that fills in the pores of the porcelain.
Finish with another wiped or spread on coat that brings the shine back out.
If you had to have it recoated with enamel you would have to have the tub taken to a company that does that and you would probably be better to buy a new tub in the first place, unless it is a free standing tub and it would still be expensive and difficult to find someone to do it.