Depends which push fit fittings have been fitted, e.g. "speed-fit" fittings can be unscrewed, "poly fit" fittings are permanent.
If there's no way of unscrewing the fitting, it's permanent, unless you break it off with a bit of force.
If the fittings attached to copper, fitting can be replaced for a speed-fit reusable fitting. If it's attached to plastic piping, stick with the same brand of pipe for the fittings.
I have to correct the previous poster Speed fit & poly fit are demountable these are WHITE in colour. Poly Plumb is a non demountable fitting and is GREY in colour. Push fit copper fittings Tectite Classic from yorkshire fittings is demountable (tool required) but Tectite Sprint is a use once fitting. No doubt there are other systems out there but as antony5378 said it depends on the make.
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