It is possible that changing elements in a painting based on an online artwork could still result in a copyright violation. Copyright law protects original works of authorship, including visual art, and gives the copyright owner the exclusive right to reproduce, distribute, and display the work publicly. This means that if someone creates a new work that is substantially similar to an existing copyrighted work, it could be considered a copyright infringement.
There are some exceptions to this rule, such as the fair use doctrine, which allows limited use of copyrighted material for purposes such as commentary, criticism, news reporting, and scholarship. However, the fair use doctrine is a complex and fact-specific legal test, and whether a particular use of copyrighted material qualifies as fair use depends on the specific circumstances of the case.
Therefore, if you create a painting based on an online artwork and change some elements, it is still possible that the new painting could be considered a copyright infringement if it is substantially similar to the original work. To avoid potential copyright infringement issues, it is generally a good idea to obtain permission from the copyright owner before creating a new work based on their original work, or to ensure that your new work is significantly different from the original.