Using Adobe Premiere Element 15, I would like to move on the 18 or 19 version. How to ensure that I will not loose my numerus éditions ? Should I cancel first 15, then upgrade to 18 ? But this will delete all my work no ?
You upgrade in Premiere Elements by buying the new edition. Buying direct from Adobe can get you a discounted price but often it is cheaper to wait for the holiday sales from various retailers.
Version of Premiere Elements exist side by side. Installing a new version does not change anything with the existing version and its projects.
Be aware that "backward compatibility" is NOT a design goal with Premiere Elements. Opening a project from a previous version may actually corrupt the project file. In other words, keep version 15 for any unfinished projects or projects you want to re-open. Use the new version for new projects.
Having done a number of upgrades, make certain you backup your catalogues locally and w/in the app itself (CTRL B). That way you can restore in case something goes off the rails. I'd scour Adobe user forums, YouTube, and any other content you can find prior to pulling the proverbial trigger on the nuances of doing this upgrade.