The WA specifically allows for people to change circumstances / designations - the person being discussed would simply stop being a frontier / S1 cancelled, become France employed (? or student?) - I don’t know what a CDD is…) for a year then recommence their frontier work with a new issued S1.
Presumably it would just be a case of ensuring the S1 dates line up with the France social security dates to ensure continuity of cover?
Well TBH John : you are not alone. As Covid has changed work patterns so fewer would technically meet the conditions to have an S1 as a frontier worker if the requirements have stayed the same as pre-Covid - i.e. the weekly commute home to France.
Plus there is the obligation for the employer from wherever, to pay full French social security charges, if their worker resident in France is now working more days remote sitting in France. There’s a tolerance of 25% of work time before this is enforced on the employer and employee - currently.
And lastly the good news is that it’s not long due to Brexit, before.there are even fewer workers resident in France that would qualify for an S1 anyway 
This might help clarify.
The EU has made specific tax and social security arrangement for those who meet the EU definition of frontier worker or cross border worker.
There are a lot of folks in the EU who live one side of a border and work the other side, commuting each day or on a weekly basis. If you live near a border it can happen that your nearest employment opportunities are the other side of that border eg Belgium/France, France/Switzerland, Germany/Austria etc so obviously it was very necessary to make these arrangements. But the same arrangements applied to UK/France commuters.
But, simply living in one country and getting a job in another does not mean that you are a frontier worker in the sense of the EU Directive. In order to be covered by the arrangements, you need to meet certain conditions. (A bit like, freedom of movement in the EU sense of the term has a special sense with certain conditions attached, )
To the best of my knowledge, the UK is no longer included in the EU frontalier scheme so cross border workers commuting between UK and say France cannot benefit from the special arrangements BUT those who were working under those arrangements before the end of transition, have their rights protected by the WA and so they can continue the same for as long as they continue to be a cross border worker with that employer on the same working pattern.
The question here as I see it is, could the UK issue a new S1 with a start date post Brexit, given that under the WA the UK cannot set up new frontalier arrangements that start after the end of transition.
yes, if you mean for citizens covered by the WA, e.g. my latest one was issued September 2022.
If you mean for people not covered by WA - I think social security co-ordination continues because of the trade agreement so that is a maybe? After all UK state pensioners retiring to France not covered by WA also get an S1 ?
The UK gov site says
" Your rights as a frontier worker
If you were frontier working before 1 January 2021, your rights are protected by the:
As a frontier worker, you may:
You can only continue to work in countries where you were previously regularly commuting to work. You cannot start working in new countries as a frontier worker."
(my bold)
So it seems to me that (a) clearly things changed for frontier workers on 1.1.21 and (b) it depends by what is meant by “you can only continue to work”. Does continue mean it must be continuous or does stopping and starting again count.
I do not suppose that when the agreement was being made anybody thought to make provision for stop start situations in which case there would currently be no answer to be found anywhere on this.
It will be interesting to see what is decided.
Do you have Facebook? There are some great groups that can help with this. Strictly fiscal France, strictly legal France and strictly sante France.
That’s the essence of the original question and I’d say here ‘continue’ does not mean ‘continuous’ the frontier rights would not be lost by a pause in activity. For example if someone loses their job then finds another, there could be a gap.
I also revisited the trade agreement - it has a protocol on social security co-ordination which maintains cross border working S1’s - so it looks like one could theoretically start in a new state but would be dependent on being allowed to work in a new state. I presume this is what is meant by the gov.uk phrase ‘you cannot start working in new countries’ - one can start but only if one has a right to work in that country, i.e. a visa.
Hi folks, coming back to this a bit later than expected. Just to clarify, the individual is a British national who is resident in France but commutes to the UK to work. It was indeed a valid status for the obtention of a CdS in the terms of the WA.
Thanks for the links, I will pass on the links to him in particular the one that says you can work in your country of residence and work in two countries at once. In his case this may apply I guess.