Only by a complete exchange-an exchange into another type should cost 15€-but a bit depending  on where you live it may take many weeks before the paper comes in the mail.

I see youre Brit-you can also order it in much shorter time on some of the few remainign UK private agencies that are also able to provide you with i, Ffestiniog travel comes in my mind, there are a few more. But then you pay the 15% cancellation fee for the mobile pass. It will/may take weeks/monthes before that money comes back.

IF you only ask this becse of this ´pass cover nr--needed in a very few cases for a RES-then forget it-it is now possible to generate one via the site.

Which again shows that giving some more info and thinking about what/why is always a useful thing

Sorry not sure what 'pass cover nr needed' means? I am mistrustful/ relatively inexperienced with smartphone (given to me by my grandson). Know I will need WiFi or signal to retrieve pass/tickets, so just felt a paper version would be more comfortable/stress free for me. Still unsure how I would go about converting to paper. We travel in 4 weeks time.....

I don't think that Interrail will have time to deliver a paper pass to you in 4 weeks so in that case you need to find someone in the UK that sells paper passes. Then you make a refund of your mobile pass from Interrail.

Have you already made your reservations for the Eurostar? Otherwise it might be good to do it soon. For making the seat reservation at b-europe you need a Pass Cover Number which you can get here.

https://community.eurail.com/news-and-announcements-39/pass-cover-number-generator-is-live-5653