A quality guitar like a Gibson should not weaken with age, I wouldn't think**. Are there obvious signs of structural damage, or any other reason to worry that normal light- or medium-guage (or even heavy) strings would unduly stress the instrument?

By "tuning to E," should we assume you mean standard tuning? If so, moderately heavier strings shouldn't be a problem.

If you are instead referring to open-E "vastapol" tuning, excessive string tension can indeed be an issue -- but one that is easily solved by using open-D instead, the exact same relative tuning except two half-steps lower. (You can always capo back up two frets to play in E).

Because you learned on the extra-lights and are so accustomed to them, I wouldn't jump all the way up to heavy guage "cables" -- you'd probably have difficulty fretting them. However, light (rather than extra-light) strings probably would not present a problem. Later, you might try moving up another notch to mediums, which would give you quite a bit more volume than you're getting now.

** As far as "what I wouldn't think" is concerned: If you have ANY doubts about the instrument's strength and ability to withstand additional string tension, by all means have a qualified person take a look at it. There's no way anyone here can offer a valid opinion over the internet