Data access and load balancer integrations of new prod environment

We would like to make a progressive transition to our new prod environment, for example:
- start with redirecting only 10% of the traffic to the new instance
- monitor the new instance
- if everything checks out start redirecting more (or all) the traffic to the new prod instance

This poses some challenges:
1. The plan is to to deploy the new infrastructure in a fresh production account
2. Because the 2 environments will run in parallel for certain amount of time, this means that they need to share some resources:
- access to the same DB
- the new prod environment will be configured as a target in the existing LB

We need a plan on how to overcome the challenges listed above. Possible options:
- enable cross-account data access for the DB for the transition period
- deploy the new infrastructure in the existing prod account, and migrate to a new account once the solution is validated