So I've witnessed two retools given the last two franchises I've covered. They were very different, but they give you an idea of how the Blues could go about things.
In Florida, Bill Zito purged his roster of all the bad contracts. He bought out Keith Yandle. He paid Arizona to take Anton Stralman's contract. He got rid of James Reimer's deal. He traded Mike Matheson. He shipped Brett Connolly to Chicago. The Panthers paid a high price to free up cap space, whether in draft capital, prospects or dead cap (with Yandle's buyout). Then they operated on the margins to find young, undervalued assets. Think Anthony Duclair, Carter Verhaeghe, Gustav Forsling and even attempts at getting production out of Nikita Gusev and Olli Juolevi. That's how they built their Presidents' Trophy team, plus having Barkov, Huberdeau and Ekblad doesn't hurt. Where they're in trouble now is they spent all their cap space and draft picks at the deadline last season and are sort of stuck, and could miss the playoffs. Zito did a good job of clearing space, but you could argue and poor job of using it.
In Dallas, they refused to deal first-rounders for rentals at the deadline and used their later first-round picks well. Even their second-rounders. Because they drafted Miro Heiskanen, Jake Oettinger and Jason Robertson in the same draft (plus Roope Hintz two years earlier), they were able to transition seamlessly from Seguin, Benn and Klingberg to their current core. Add in prospects like Wyatt Johnston and Logan Stankoven and the Stars are set up well.
When I look at the Blues, they're probably closer to the the Panthers given their contract situation. But there is some hope for the next wave of prospects like Neighbours, Bolduc and Snuggerud. The only way towards a retool is hitting on draft picks consistently, and if the Blues can hit three years in a row, that's a start.