Converting RGB procreate file to photoshop CMYK?!!

I didn't realise all my canvases I'd set up were in RGB,

Which RGB Space? 

What kind of print process are you looking for? 

For fine art printing digital some printing devices (which can employ far more than 4 inks) can offer huger gamuts than any CMYK-process.

On a comparable substrate that is, comparing a print on canvas to a CMYK-print on glossy paper would naturally be pointless. 

It appears that Procreate supports CMYK, so you can copy/paste the RGB file into a new CMYK canvas that uses the appropriate CMYK ICC profile:

https://ipadcalligraphy.com/procreate/convert-to-cmyk-in-procreate/

I believe that the iPad version only supports RGB mode.

When you state that the printers won't accept RGB, I presume that you are referring to print service providers?

Do you have a desktop computer or laptop?

If you have a Mac, you don't even need Photoshop as you can convert to CMYK using Apple ColorSync, but not edit, so limited. There are of course other image editing software which may support CMYK conversions and or softproofing etc.

Otherwise the full Photoshop on the desktop/laptop for Mac or Windows will allow you to softproof and edit for a CMYK conversion and or edit the results of a CMYK conversion.

Keep in mind that there isn't a generic CMYK, so you need to know what type of CMYK profile to convert to (what press condition).