Please check reopen.europa.eu as a starting point for such questions. There are extra requirements for passengers arriving in France from the UK.
You can change your Eurostar reservation but only if there is still availability in the new train you'd like to use. Details about how to do that should be in the confirmation mail you received for the Eurostar reservation.
There is still some availability for Saturday evening and also on Sunday.
Reopen.eurooa.eu points to:
https://www.diplomatie.gouv.fr/en/coming-to-france/coming-to-france-your-covid-19-questions-answered/article/coming-to-france-your-covid-19-questions-answered?var_mode=calcul#sommaire_3
So you need pressing grounds to enter France from the UK. Not sure if that also applies if you're only passing through the UK and not staying there. Sounds like you'd better avoid the UK.
I am in France and am aware of pre and post Covid tests and locator form to enter the UK. It seems there is a lot of confusion and ad hoc decision making by local officials. France announced on Thursday that all but essential travel would end the next day without making it very clear. All the trains on the last day today were full. We’ll see what happens.
Rules te ENTER a certain country are set by that country. So not really by FR to enter UK-BUT I assume you dont want to stay in UK forever (unless you are from there?) so youĺl have to comply with the-by then probably new again-rules of the country you enter after that visit.
INside EU rules are more or less same for all citizens, but as you know UK wanted to go out of it. But at the mo confusion is rife and rules change by the minute-and usually for the worse.
I’ve taken the Eurostar with an Interrail Global pass, the day this question was asked. At that time, a major reason (motif impérieux) was needed to enter France from UK. Being french (or being married, etc) is one of those reasons. And if you are just passing, staying less than 24 hours for changes to travel to another country, you probably don’t need to be french.