Well I don't really like your assumption saying 'If you were born a peasant you died a peasant', this is a little bit more complicated than that... As for the second question, this also very complicated, rich people, let's say the nobility, would not devolve to the 'peasant class' even if they were to lose their wealth...

Let's talk about 17th/18th century in the north-western part of France (Maine/Anjou/Brittany). The nobility class there was well representated and generally poorer then the nobility in the rest of the Kingdom at that time. It was not uncommon to see members of the french nobility working in the fields. Something that was not allowed per se due to their class status but common anyway. But still, they were not part of the peasantry even if some of them did struggle with money.

I'm going to give you a good and very specific example I came across during my studies. I used to work on pieces of archive related to a french noble family, the Knights de Meaulne. One notable member of the family, Claude III de Meaulne Lord of Bourny (1669-1748), did spend a fair bit of the family wealth up to the point that he even was beaten up by peasants for 'putting shame to his position'(!). This good sir lost his nobility TITLE when his brothers and sisters did put a claim on his estates but he did not lose his nobility STATUS. He ended up broken but still a member of the nobility. The interesting part being : he later married a commoner, which put an end to his noble line.

As for reference, Annie Antoine did explain this part quite a bit in her book Fiefs et villages du Bas Maine au XVIIIe siècle the interesting case of 'poor noblemen' in this part of France prior to the Revolution.