Shower hot and cold directions?

We have just had a shower fitting replaced - not electric but when we turn it on hot and cold are opposite directions from indicated on valve ..plumber has told us is because original plumbing in wall is wrong way around ? If this is so why did the previous valve work fine??
Thanks

Advice from 1 local tradesperson

or maybe the new valve is upside down ? is it a bar mixer
unless the old one was not a thermostatic one

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