Pretty sure you can't. All these contracts are just pre-contracts. Riders just don't go back on them, and other teams look at them as legit.
As far as I know, the actual recruitment (signing the rider) also has to take place between the beginning of August and the end of December.
Per GoogleBard AI...
What is the UCI rule on riders signing with men's elite teams? Can they sign before August 1?
No, riders cannot sign with new men's elite teams before August 1. The UCI's Rider Transfer Period for men's elite teams runs from June 1 to July 15. Riders who are not under contract with a UCI team on July 15 can sign with a new team starting on August 1.
The UCI's Rider Transfer Period is designed to give teams time to plan for the upcoming season and to prevent riders from jumping from team to team at the last minute. It also helps to ensure that all riders have a fair chance to find a new team if their current contract expires.
There are a few exceptions to the Rider Transfer Period rule. For example, riders who are released from their contracts by their current teams before July 15 are free to sign with new teams immediately. Additionally, riders who are under contract with a UCI team but are not racing for that team due to injury or other reasons can sign with a new team starting on August 1.
If you are a rider who is interested in signing with a new men's elite team, you should contact the team directly to inquire about their transfer policy.