In my language we have an expression which literally translated is “He was born a fireman but died in a fire” as a nice methaphor for a big shift in your way of thinking (like from conservative to reactionary). How would you say that in an elegant English?
By the way, the original expression in Italian is Nacque pompiere ma morì da incendiario (and you can say also the other way round).
We have the expression :
poacher turned gamekeeper.
It is used in various contexts where someone changes from one position into the opposite, though often it is when someone assumes an authoritative position (the gamekeeper phase) who once was in the position which was subject to regulation (the poacher phase).
1.BRITISH someone who now protects the interests they previously attacked. "in the clearest case ever of poacher turned gamekeeper, the brewers are joining together in a crusade against alcohol abuse"
University poacher turned gamekeeper sets sights on vice-chancellors’ pay
The Guardian 10th October 2017
The Ngram shows stronger usage, BrE over AmE, and dates the saying from about 1900.
The examples people have given mean the opposite of what you want to say. I cannot think of an idiomatic phrase in English with that meaning in English. I would use a near-literal translation "a firefighter turned arsonist" in inverted commas and then explain just what you mean.
You mean something like these?
If You Are Not a Liberal at 25, You Have No Heart. If You Are Not a Conservative at 35 You Have No Brain
(from Quote Investigator, which gives other examples)
Or another
He was a liberal until he got married and had kids, after which he was conservative.
