In all seriousness, is there such a thing as too old to give it a try? 

Absolutely not! Just take your time and start slowly adding speed once they're "patterned" going to the left, most of them pick it up pretty quickly. Any particular reason you're switching? I actually prefer one that can run either way because certain setups you have an advantage one way or the other if the alleyway is off center.

Over the years I've noticed this horse when left out in pasture, tends to graze with the front right forward, and usually picks up her right lead when out on her own, if she's playing, etc. 

Also, I feel like her 1st barrel is the best of the 3. She's no spring chicken....in her teens. But she's very broke, so I think she'd pick up on it quickly. I just wonder since she's been patterned and solid for over a decade, if it's just too late. I don't want to confuse her or tick her off if it's past the point of being able to teach an old dog new tricks, you know?

Not to mention, there are some arenas in my area that it would greatly benefit us both if she ran better to the left. 

I switched my mare at 12 years old and it was a great decision. She's 15 now and still running left. It won't hurt to try!

Switched my old bay mare when she was 14 after she fractured her left hock to left barrel first.  I honestly think if you are changing them for the right reasons (aka not tjust putting a bandaid on a lameness/training issue) the switch is pretty darn easy.

Nope no bandaids here. Old gal has been properly vetted and gets chiro and rechecked 4x a yr. This mostly is coming from me watching her over the years and going "why didn't I see if she was a lefty years ago?" 

She'll probably do just fine.

 If it ain't broke ... don't try to fix it ... 

I honestly can't believe GBean hasn't been here... Where is she?? She is the go left gooruu

Replying to the if it aint broke don't fix it. 

So, I went out when I had a dry enough day just to trot a few patterns to the left first. She made it explicitly clear she didn't enjoy me switching things up. Not in a  bad way, I've just had her so long I could tell she was annoyed and very much "seriously?" that I was even doing this silly thing. So I said ok, let's not fix what isn't broken, old girl, trotted a pattern to the right, patted her neck, put her away.