Hi David (@tenuki),
Welcome to the community 
You may want to migrate existing projects from @openzeppelin/upgrades to OpenZeppelin Upgrades Plugins.
See: Building for interoperability: why we’re focusing on Upgrades Plugins
If you were deploying using the OpenZeppelin CLI you can migrate to Upgrades Plugins using the migration tool: https://docs.openzeppelin.com/upgrades-plugins/1.x/migrate-from-cli
Proxy contracts are also part of OpenZeppelin Contracts: https://docs.openzeppelin.com/contracts/3.x/api/proxy
Can you describe a bit more your use case and what customization you want to make?