There are two or three small tits formed into the back of each letter.
These tits are about 1/8" in diameter and about 1/4" long. Without removing the endcaps you will not be able to remove the letters without knocking these tits off.
To remove the letters get a good putty knife and work it under the letters, then shear off the tits with a good wack to the knife - much the same procedure used in removing rivets on the roof without having to drill them out.
You might have to seal the holes caused by the original mounting holes. I remounted my letters (on the front) by drilling holes in the letter and using Olympic rivets to hold them on. I was able to drill out the tits as there were slight casting indentations on the front indicating where the original mounting holes were.
When I remounted the letters I put a good film of vulkem on the back of each letter to make sure no water would be trapped behind them. Be sure to seal the Olympics with a daub of Vulkem also.
I removed the front letters and painted them, then replaced them as above, but, on the rear I repainted them while they were still in place as per the picture below.
I polished the edges of the letters (under the tape) prior to taping them up and etching the rest of the casting. Paint is Rustoleum "Royal Blue"