The phrases poacher turned gamekeeper, an old poacher makes the best gamekeeper, and variants, denote a person who now preserves the interests that he or she previously attacked.

One of those phrases occurs in Boris’s real problem? He is competitive and no one’s challenging him, by Tom Newton Dunn, published in the Evening Standard (London, England) of Wednesday 6^th October 2021:

I worked out what we were missing: the Boris Johnson character of old who terrorised Tory conferences for a decade under previous party leaders.
Whether as London mayor, foreign secretary or rebel backbencher, he’d appear for a day (usually Tuesdays, which we dubbed “Boris Day”). A conference darling and a challenger to the throne, hoovering up all the attention and wreaking havoc on No 10’s carefully constructed news grid. An ever-present threat keeping David Cameron and then Theresa May on their toes.
Now the poacher has turned gamekeeper as the PM, and he is much enjoying all his new clothes.

The phrases poacher turned gamekeeper, an old poacher makes the best gamekeeper, and variants, are first recorded in the 19^th century, but the notion had already occurred in The Physician’s Tale [lines 83-5], by the English poet Geoffrey Chaucer (c.1340–1400):
—text and interlinear translation from Harvard’s Geoffrey Chaucer Website:

83 A theef of venysoun, that hath forlaft
     A thief of venison, who has abandoned
84 His likerousnesse and al his olde craft,
     His greedy appetite and all his old craft,
85 Kan kepe a forest best of any man.
     Can guard a forest better than any other man.

Very similar to an old poacher makes the best gamekeeper, the phrase the greatest deer-stealers make the best park-keepers occurs in The Church-History of Britain; From the Birth of Jesus Christ, Until the year M.DC.XLVIII. (London: Printed for John Williams, 1655), by the Anglican clergyman and historian Thomas Fuller (1608-1661):

Campian catcht by Walsinghams setters.
Secretary Walsingham, one of a steadie head (no more than needfull for him, who was to dive into such whirle-pools of State) laid out for Campians apprehension. Many were his lime-twigs to this purpose. Some of his Emissaries were bred in Rome it self. It seems his Holiness was not infallible in every thing, who pai’d pensions to some of Walsinghams spies sent thither to detect Catholicks. Of these, Sled and Eliot were the principal. Surely these Setters could not accomplish their ends, but with deep dissembling and damnable lying. If any account such officers evils, I deny it not, but adde them to be necessary evils, in such a dangerous juncture of time. Alwayes set a—to catch a—;and the greatest dear-stealers, make the best Parke-keepers. Indeed these spies were so cunning, they could trace a labyrinth, without the guidance of a clew of thread; and knew all by-corners at home, and abroad. At last Eliot snapt Campian in his own lodging, and in great triumph he was carried to the Tower.

The earliest occurrence that I have found of the phrase an old poacher makes the best gamekeeper and variants is from the following by the English author and political reformer William Cobbett (1763-1835), published in Cobbett’s Weekly Political Register (London, England) of Saturday 14^th February 1807—however, William Cobbett implies that the phrase was already well established, since he refers to it as a “maxim”:

The circumstance of Mr. Rose’s name being amongst those of the petitioners against ministers for using undue influence, has excited some surprise; and, I think, not without reason. Had he, indeed, himself ever been, when he was in Mr. Freemantle’s situation, in the habit of being the agent of undue influence, one might have accounted for his now coming forward, upon the philosophy of the maxim, than an old poacher makes the best of game-keepers; but as all the world knows, how scrupulous he was upon all such points; how rigidly he adhered to those pure principles of his great patron, which procured for the said patron the surname of heaven-born, one really is at some little loss to guess at the cause of his having been selected as a leader in an enterprize of this sort.

The earliest occurrence that I have found of the phrase poacher turned gamekeeper and variants is from The Atlas. A General Newspaper and Journal of Literature (London, England) of Saturday 13^th December 1851:

THE JUBILEE OF 1852.

The Court of Rome, whether elated by the peculiar glories of its actual position at home, or with the singular happiness of the position of other countries abroad, is about to organise a jubilee for the year 1852. […]
[…]
In this age of patchwork, it is probable enough that the jubilee will serve its purpose. The tone which its celebration will give to the European mind will be strong enough to last a year or so. It will, besides, give an opportunity to the despotic powers of disembarrassing themselves of many of those troublesome incumbrances—political prisoners and political refugees—the latter more especially dangerous while smarting under exile, and amidst the facilities of plotting afforded by a nucleus of desperate spirits, and the refuge of a free country, may be turned into very harmless citizens by a judicious restoration afforded under a sufficient plea. Many of them will be even serviceable to their forgiving sovereign, on the known principle of the poacher turned gamekeeper.