I found this list on the Dell site that tells you whether they have tested a particular model for its ability to be upgraded to Windows 10. The Dell Vostro 3550 is not on the list of about 20 Vostro models, some with lower model numbers , some higher. Will they ever test it??
If you try to upgrade, windows 10 copies files and then in the 2nd of the three major sections of the upgrade (where you have that big circle in the middle of the screen) it just goes to a blank screen and then all disk activity ceases and it becomes well hung. Of course now, the machine thinks its os is win 10 and you cannot go back to a win 7 recover point . So you just lose all your data and apps and restore the factory image and then install 208 windows 7 updates and it takes like 8 hours in all.
Sounds like a similar problem that people had with Vostros going from win 7 to win 8. Why not fix it.