Grant,
I would imagine you will want to remove as much of the "peaty mix" as possible as to not introduce a difference in the mixes drying time. The 511 mix will obviously dry faster than the peaty mix and while you may believe the plant is dry enough to water again, the center of the root mass may still be moist/wet.

I failed to remove all the "peaty mix" from a ZZ Plant I purchased last year and planted it in fast draining cactus mix. When the cactus mix would dry, I'd water. Almost a year later, the ZZ Plant died from rot. While the outer mix was drying out fast, the dead center of the root mass was staying too moist and it literally rotted from the inside out.

Planto

I agree with Planto, remove as much soil as possible. I bought some new TC before Christmas and I have had problems. I did not remove all of the soil because they were in bloom and I did not want to disturb the roots (they were shipped out of a pot but with soil). I have had rot in two of the plants and plan to repot them this weekend. Good luck.

Valerie

Please do remove all the peat from the roots. This is my first TC after I repotted it. I did not remove the peat because I, too, did not want to disturb the roots. The leaves got dessicated and brittle in spite of my watering.

The folks here convinced me to repot a second time. When I turned the plant out of the pot I found large sections around the roots that were dry as a bone. The peat could not take up water. So my problem was the opposite of peat holding too much water, but a problem just the same.

Here it is this past Thanksgiving.