Each item type has a different limit to safe upgrading, past this point of upgrading you have a chance of destroying or wiping the item.

Normal items – White

You can safely upgrade to +4

Type A items – Purple

You can safely upgrade to +3

Type B items – Yellow

You can safely upgrade to +2

Type C items – Orange

You can safely upgrade to +1

Relics, Leons, Special etc.

Just don’t. Convert the item to Type C before attempting. Not only is there no safe limit, your odds of successfully upgrading are poor.