If you have an Access Server with fixed license keys, and you activate a subscription on it, the subscription will take over. This means that the fixed license model and the subscription model cannot work on the same server simultaneously.
However, an account can have both models. You can have the fixed license key and subscription key on the same account as long as they are activated on different machines.
If you have 5 fixed license keys with 10 licenses for each key (50 total licenses) activated on server 1 and decided to activate the subscription key with 10 connections on the same server (server 1), the subscription will take over, and the fixed license keys are suppressed. With these, you can only use the 10 connections under the subscription key and the 50 licenses from the fixed keys will be deactivated.
You can read more about the subscription model here: Subscriptions FAQ
Make sure to activate only one type of license per machine (server).
If you encounter any problems in the activation process, please refer to this page first: Troubleshooting problems with software licensing.