Of course you can purchase and activate a PRO license while using the trial version.
However when you do so, you will be using it at the same time as the 42 day period for the trial version. In other words, the permitted use period for the trial version does not come to an end even when the PRO license is registered. The trial version can only be used for 42 days after its use was started.
MEMO
During the trial period (42 days), you can continue to use the trial version even if you deactivate the license. Note
The license period (1 month, 1 year, 3 years) starts from the moment of purchase, regardless of the authentication status.