Downtime if upgrade to fibre?

It looks like my old ADSL router is on the way out. As I'm out of contract, Plusnet will charge me to supply a new one - or I could just go on eBay, which looks like most people's recommended way. But I probably might as well bite the bullet and upgrade to fibre anyway. The only hitch: I need to submit an academic assignment at the end of March, and can't risk being without internet. If I place an order for the upgrade, (a) What is the likely lead time, and (b) how long, if at all, can I expect to be without internet as the switch takes place?

A friend of mine went from ADSL to FTTC with Talk Talk and there was little downtime, but then Talk Talk seems to use the same router somehow.
It is pretty quick these days.

In reply to a post by TLM:

It looks like my old ADSL router is on the way out. As I'm out of contract, Plusnet will charge me to supply a new one - or I could just go on eBay, which looks like most people's recommended way.

If your throughput speeds are currently adequate for your needs then stick with ADSL for the time being. There are several pre-owned Billion BiPAC 7800N routers for sale on eBay for around £15 to £20 at the moment...

Upgrade to FTTC .... the downtime should be about half an hour, often less.

Do you have a phone with data that you could use to tether if there was a short period of downtime?

No.

Strictly speaking, for the purposes of the assignment at least, it shouldn't matter even in the unlikely event it was down for a day or two. As long as the outage was not on or immediately before the submission date of 30th March. That's all I was a bit worried about, really - Covid issues contributing to a long lead time, and then the internet going down the very day I'm trying to submit the work.

There's always McDonald's free wifi !