If I sold some bonds within my 401k and immediately purchased new bonds within that 401k, does that qualify as taxable income?

Transactions within a retirement plan are not taxed. 

No.  You don't report transactions inside a 401K or IRA.  Only withdrawals and rollovers to another account like to an IRA.  All the gains or losses stay inside the account.  That's  one of the benefits of having a 401K or IRA.  They can grow a lot and you only report it when you take a distribution.  But then it is taxed as ordinary income and not at capital gains rates.