One should use a 3PDT relay = 3 poles double throw relay.
Reason: protection in case of relay breakdown.
I suggest that multiple relays are not used unless they have a second independent set of contacts to insure all relays are energized and working.
I have an automatic switch over based in output voltage and current, but it is in a PC with problems.
The mill starts in STAR configuration, then once the current is certain value, the relay is energized for DELTA configuration and the circuit needs to take in account the current charge increment/decrement to maintain the configuration in the proper setting.
The main problem is the current detection for configuration setting, for heavy currents I have used the Allegro 75 & 150 Amps current detectors.
Nando.