How fungible are personnel salary allocations in an NIH grant?
Generally I think yes, but definitely talk with your institution/department's grant staff about this even if you're still in the writing phase - they'll be your best friends.
For example (from NIAID but I don't think this is Institute-specific):
https://www.niaid.nih.gov/grants-contracts/putting-effort-your-application

Don't worry if your effort changes over the course of your grant. Reviewers know that can happen, and NIAID allows you to change your effort as your work progresses (with some limitations we'll discuss in a future issue).

One of the exceptions is substantial reductions in effort for key personnel:
https://grants.nih.gov/grants/policy/nihgps/html5/section_8/8.1.2_prior_approval_requirements.htm

A significant change in the status including but not limited to withdrawal from the project; absence for any continuous period of 3 months or more; reduction of the level of effort devoted to project by 25 percent or more from what was approved in the initial competing year award.

You'll have some opportunity to firm up the budget if your award is funded and during the extensions process.