Changing of iMessage account
You can have separate accounts for iMessage and iTunes purchases. Having a different iMessage account will not affect your purchased apps.

To change the iMessage account, go to Settings -> Messages -> Send &amp; Receive. Tap on your current account name to change it.

For iTunes purchases, your account is specified at Settings -> iTunes &amp; App Store.

You can even have a separate iCloud account.

The only caveat with doing this is that when setting up a new device, you need to be sure to enter these accounts separately. If you follow the default setup flow, both Messages and iTunes will use the same account (but you can change it as outlined above.)