There should not be an increased chance of infection when removing an existing chin implant and replacing it with a new one.
Removal and replacement of a chin implant can be performed under local anesthesia as an outpatient surgical procedure. The removal of the old implant and placement of a new implant should not have any bearing upon the infection rate. In our practice, we use antibiotics when performing implant surgery.
Having a prior chin implant does not place you at greater risk of infection should you make an exchange for a new larger chin implant. The important questions about your next chin implant is what is the type and size of your current one (from 20 years ago you are unlikely to know) and what dimensional changes (horizontal, width and vertical) do you want to see in the new one.
There is no reason I can think that would make undergoing a second chin implant would make you higher risk to contract an infection. Other questions I would consider:
1) What type of implant was originally placed?
2) How was the chin implant secured? (are there screws?)
3) How is the sensation of your lower lip? Are there currently any areas of numbness.