Heck, by golly gosh yes!
A DM should alter that stats/stories/etc. of anything he sees fit, even if using a published and official game world, to tailor it to his campaign. Otherwise, what's the point?
Take my FRCs campaign f'rex. Now, my players will want to stay out of the next paragraph or two.....
I decided that Larloch was the First Lich. Why? Where does it say that officially? Nowhere. I just decided that I liked the idea. He was also the lover of one of the founders of the Eldreth Valt..ah..whatever, the name of that elven supremecists group is...who founded it in part because she was jilted by Larloch way back in Netheril and wanted a little revenge.
Is the organization that old? Well, canonically, it dates back to "only" Myth Drannor. But, at least the seeds, or proto-Eldreth, was already founded, partially by the NPC. After she was freed from her imprisonment, she decided she wanted her organization back so she killed all the other leaders of the Eldreth (trust me, she had the power) and claimed the organization for herself.
Death is a pretty big NPC change, especially when not at the hands of the PCs. So now, the PCs have a thousands-of-years-old lich heading an organization that is growing in power on an almost daily basis (Epic Leadership, here I come...) who knows about the PCs (well, three of 'em anyway) and will come gunning for them soon, and, if the PCs fail to stop her current plans, will deal a destructive blow to Waterdeep.
Much more interesting, IMHO.
I cannibalize the good stuff, rework it and regurtitate it as even better.
But, that's just me, of course, YMMV.