If they fail miserably at the lead role, they will become disillusioned, maybe face a PIP if they fail bad enough, and best case will end up as a more grumpy IC that feels like the company screwed them over.
If they succeed, then you will almost certainly be unable or unwilling to pay them what some other similar company would pay them now that they have proven their ability as a lead. They will stick around for a while, but once something starts to go wrong anywhere on their team or in the company (and it will) they'll realize that it's probably going to be several years before there's any potential to climb the ladder any further, and they start to sniff around for what they should be making, and you'll lose them.
It's a valid choice to "burn" personnel this way because you need a project done and can reasonably hope that the person you're burning can get it done.
But it's generally less costly/risky to either a) fight the hard fight of getting an existing employee's salary in line with their new role immediately when they start it, or b) hire an external lead and fire quickly if they suck. The benefit of an external lead is that if they're really good, they might have director potential, which is even more valuable to promote someone into, and much more politically viable for someone who hasn't just been promoted to a lead/manager.
On that note, a warning to C-levels of all shapes and kinds: when you promote someone internally from lead to director,...