I'm level 52.
So far in my 140 hours of game play, I've been a sneaking archer/assassin (dual blades)
And now that I've maxed out Archer and sneak, I want to change to a pure heavy armour melee.

Now my one handed, heavy armour and block level will be considerably lower.
And my opponents will be a considerably high level.

Will I be too far behind to be able to keep up?

Any other tips will be helpful.

Start slow; make a point of using your sword more, always wear heavy armour...and make sure you max out your training slots every level with one-hand (your Heavy Armour will shoot up on its own pretty quick just by getting hit). If you have a few spare perks, toss them into the armour-increasing perk to help offset the low level if you can, but by no means is that mandated in my opinion - just very, very, helpful. Within 4-5 levels provided you level One-hand and Heavy Armour almost exclusively you should be more or less on par. Oh, and of course, buy the best you can in both, and reforge/upgrade the hell out of them both.

Start a new character, or you could use smithing to your advantage to boost your stats.

Hmm, okay thanks for the pointer!

I'm not ready to start a new character yet.

I'd make sure to hit up the Warrior stone too to give the boost to the skill leveling - in case you didn't think of that.

Oh, yeah I forgot about that, thanks!