Is a user legally allowed to make changes to available plugins according to his needs? e.g. changing UI, code etc but not the licenses and other info.
Short answer: Yes.
QGIS and its plugins are licensed under GPL (GNU General Public License), version 2 or later. This license allows anyone to modify QGIS and its plugins, as long as you follow the specified conditions. See the full text of GPL for those conditions.
Licensing requirements for QGIS plugins are explained on this blog post.
QGIS is Open Source Software and provides a great platform for third parties to distribute additional functionality to users through our plugin system. QGIS is licensed under the GPL version 2 or greater. ... Under the terms of this license, it is a requirement that all plugins distributed via http://plugins.qgis.org (or through other repositories that may be self-hosted) should comply with the GPL version 2 or greater license.
Follow this link for the full text of GPL (GNU General Public License) version 2, as provided in the current (version 3.4) QGIS User Manual. Here's what it says about modifying programs:
You may modify your copy or copies of the Program or any portion of it, thus forming a work based on the Program, and copy and distribute such modifications or work under the terms of Section 1 above, provided that you also meet all of these conditions: (see full text of GPL version 2, section 2 for list of conditions)