Can an already dedicated line (110v) be converted to a 230v20 line by just changing the outlet Plug & cover plate?

Generally yes, but another important thing is changing the breaker.

How many wires are st the outlet? You need to have 240 VAC available at the outlet.

Do you mean 230 (240) volt 20 amp? Like a window A/C plug? If so, the answer is usually yes. However, you will have to change the breaker to a 240 bolt breaker, and take the neutral (White wire) off the bus bar and attach it to the other side of the new 240 volt breaker. Be VERY careful, you should turn off the main breaker when working in the panel. It is also a good idea to have an experienced person do this if you are unsure.