I don't see a problem with it in general, some things to consider.
1: why was metal conduit being used in the first place? are those reasons (if any) applicable to the section where plastic conduit is or would be used?
2: has appropriate provision for earthing been made?
As said, no real reason why you can't use plastic conduit, if done correctly, though I can't help thinking metal conduit would be a better job for the sake of consistency.
Haven't you got any short lengths of metal conduit hanging around in your garage?
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