Me, too...started on guitar, playing in an officers club in Alaska at 10. Added drums, upright, then bass guitar, mandolin, tenor banjo, harmonica. Then, as my longtime B-3 player/partner slowed down his lead lines because of arthritis, got a melodica, then a Wirlitzer electric to play keyboard solos. Playing complex chord patterns on guitar really helped cut the learning time on keys. One thing I did wrong was finger my chords the way I did on a guitar. That meant little or no use of thumbs. One night after I had been playing five year in clubs, a piano teacher, who came up to see us often, came around to watch from the side of the stage. She said, "my God, son, what happened to your thumbs?". With a little time and adjustments, things became easier.
I think it is generally easier to go from guitar to piano, instead of the other way around, since you have a basic understanding of chord structure, scales, etc., and the physical issues of learning to handle string pressure aren't there.
I also think that lessons (instead of the way I did it) are the way to go.
Playing multiple instruments makes the world a more interesting place. And, there are more job opportunities.
Russ