How to dynamically switch one ethercat servo motor from spindle to axis ?

If the spindle is controlled with velocity commands then it may be just a case of switching to a position PID (which still outputs velocity commands)

wiki.linuxcnc.org/cgi-bin/wiki.pl?SpindleOrient

Possible solutions will depend on the hardware you are running, so you might want to ask this question in the EtherCAT section.

I wouldn't use a VFD for a C-axis. A VFD might be able to orient well enough for a tool changer but the dynamics and precision of a rotary axis are much higher and a servo motor would certainly be my choice.
There is also a video in german:
forum.linuxcnc.org/10-advanced-configura...able-issues?start=20
I have successfully used a servo motor for this in an experimental setup. In that case I dynamically switched between step/dir position command for rotary and analog (0..10V) for spindle velocity. A modified orient component and M19 command was used to rehome the spindle when switching from spindle to rotary mode. In the end the problem was that my servo motor setup was not stiff enough in rotary mode and the motor brake had too much play when engaged.
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I gather that traditionally there is a motor with belt drive for the spindle mode and a different motor on a worm gear that is mechanically engaged for the rotary mode. Although I would be somewhat surprised if that was still the case on modern machines.

though the sample hal config in the comment ,I found a little too complex for me

Also a bit dated, a fair bit has changed since. Although setting up the spindle orientation is certainly still not the easiest thing to do.