It's a little hard to tell the scale from the photo, but if the trunk is about 1-1/2" in diameter, you could topwork it with a cleft graft (see the video link).

If you're too chicken to do this you could do the same with individual branches, but you'll eventually end up with crab branches sprouting from the trunk intertwined with your new variety.

Here is a link that might be useful: Topworking Video

I would cut the top off and cleft graft the tree. Even if it doesn't work, which is unlikely, it will not kill the tree, only promote sprouts

the tree look like it is fairly well branched and still short.

I would do 4-5 grafts on the existing scaffolds and leader. all one variety, or one for the scaffolds and something else for the leader, all different is pretty tough to manage unequal vigor.

whip grafts if the size works close to the trunk, cleft if the branches are too thick.
any crab watersprouts will be easy to clean out.

I think it will look cleaner sooner and takes advantage of the structure that's there already.
But a cleft near the base will heal pretty fast and grow with a lot of vigor and less work for a single variety tree.

I agree with Windfall for quickest results use current structure but lose the largest branch- its' excessive diameter will lead to a real imbalance.

As Windfall says, cleft will work fine as well but I believe you'll lose about a year.

Thanks for the advise, I was leaning towards using the structure already there and doing multiple grafts, glad others agree :)

Thanks again for the input!

I agree with you all to use that structure, its not bad
I would multi graph as I have seen better results
the cleft graft I did last spring... maybe too late in the spring
leafed out but did not thrive.
However the whips I pruned then graphed took off
for whatever reason the cultivars or the rootstock I'm not sure
its filling in much better than the cleft