I have add 4 servers to farm, 1 with central administration type. Have forgot to set Application servers roles for two servers. Need to change type of 2 servers to Application Servers. Is it possible or i need to remove them from farm and rejoin again with prior type?
In SharePoint 2010, the only difference between a "Web Front-End" and "Application Server" is what services you enable on the server. The bits installed are the same.
Here is an MSDN reference on the services that you would enable/disable to differentiate them.