I feel a need to change server role from Front-end to Application. Would it have any impact on underlying running Applications?
No, it shouldn't. You should be able to switch from App to Web with little issues. You will need to start the APP services and change your DNS entry to point to the new WFE. Other than that, there really is not much difference between the two. The APP server runs application services, and the WFE handles all the web traffic. These are interchangeable. You will need to start/stop services on each server depending on the roles you want the servers to have.
It will cause the issue if you are using one of the following Services
- Access Services
- Access Services 2010
- Microsoft SharePoint Foundation Sandboxed Code Service
- PerformancePoint Service
- Visio Graphics Service
These services are associated with FE role, if you try to start it on Application Role server then it will be stopped. But if you go to the Custom MinRole then everything will work.
Converting role from one to other is not a major deal, but make sure you do it off hours as it will cause little interruption. Other than that you will be ok.