Photoshop and Epson P800

Your custom paper sizes are not controlled by Adobe but by the printer driver. As suggested you will be best served if you make a screenshot before updating the driver and re-create them manually.

If you need further assistance, then this is a matter best explored with the printer manufacturer, as it's their driver software at issue. Surely it would be possible for them to retain custom pape sizes through updates but it's something. they will have to program in. 

I hope this helps
neil barstow, colourmanagement net :: adobe forum volunteer:: co-author: 'getting colour right'
google me "neil barstow colourmanagement" for lots of free articles on colour management

You can't get them back from an update, they are part of the driver. Best to make a screen capture and simply recreate them again.