These guys have been in business since 1994 and online since 2006. Their one and only online focus is to replace lenses in your existing glasses.
Chances are you will only hear about them from word-of-mouth. They do not heavily advertise, but many people have used them successfully throughout the years.
Lens replacement starts at $29 but they have additional lenses/materials you can upgrade to for more. They advertise Shamir, Varliux, Essilor, Seiko, Sola, Zeiss, and Younger Optics among some of the progressive brands. I should mention they are BBB accredited and have an A+ rating. That may help you feel more secure about sending your priceless frames to them to get new lenses put in.ReplaceALens website
As far as when online glasses shops started getting into the reglazing business: SelectSpecs was one of the first.
You are looking at approximately $40-55 for the reglazing service although they seem to have a few options available.
Due to their popularity: if you have used them before you might feel safest going with them. They are located in the UK so that may affect your shipping time positively/negatively depending on where you live.SelectSpecs reglaze website
One benefit to the service offered by 39DollarGlasses is that the prices follow their regular prices (starting at $39 for polycarbonate) so it's pretty easy to figure out; and a number of the lens upgrades they mention included branded lenses like Seiko, NuPolar, DriveWear, Transitions, SMARTshades, Illuma, UltimateView, etc.
They also mention that they will ultrasonically "clear" your old frames if you are from the US, which I am assuming means clean.
You should check them out if you want polycarbonate or brand-name lenses put in your old frames. They are BBB accredited with A+ rating.39DollarGlasses re-lensing website
A lot of online glasses stores will actually do a relens for you if you inquire. It is not a commonly requested feature so many do not advertise it or they did but have not kept the page updated (for example I would have listed Goggles4u but their page is wonky and it ends up charging you $47 instead of the $26 they claim).
So if you have ordered from an online place before you might want to inquire about a re-lens if you were very happy with the service and product.
Do keep in mind that sometimes bad things can happen though. Some frames get brittle as they get older so if your frames are very precious: make sure you get from a place you are comfortable with!
<< These wasn't the relens you were looking for *hand-wave* (take me back to the start!)
Some data might be wrong on this site. Do not take it at face value. Do your own research to verify the claims here.