The software binaries, used for the various levels of software (i.e. Trial, WebPack, Design Edition, and so on) are the same.
Therefore, you can change the level of the Xilinx software at any time by acquiring a license of the desired level from the Xilinx website.
If you did not run a full installation initially, make sure that you install the device families and applicable software for the selected software level prior to using the software.
For information on obtaining a license, see (Xilinx Answer 42396).