how to change controls on Spektrum DX6i?

[JensR]

Why would you switch the signal?
That means your switch needs to be able to switch each signal - you'd need a 4 pole, double throw switch.
If you switch minus or plus, you only need a single pole, double throw one - easier to get, easier to wire up.
Or am I missing something?
Are you using some kind of electronic switch, like the ones from Servonaut or CTI?

You could use a servo-actuated switch, too. And then it is (IMO) better to use the SPDT switch.

One thing to consider: If you reconnect a servo or ESC, it will "jump" to the position/speed the stick demands, which might be different from its current position.