While every job evolves over time and you should never expect to be doing exactly the same thing from year to year, any time there is a material step change in your workload, responsibilities, or circumstances you should be very ready to press for an increase in salary that goes hand in hand with the job you are doing, impo.
That includes:
- reshuffles where they change your job title. Hey you can bet your ass that some people are getting payrises off their fancy new job titles, make sure that you know the game and are included in the conversation
- any time there's an acquisition where you've haven't been laid off - hey, they didn't lay you off, must mean you know something valuable, so now you have leverage
- Any time you have to absorb some of the workload of someone who's left or moved teams and hasn't been promptly replaced
- Any situation where workload or responsibilities have been increased to you in as you gain more expertise
Sometime this can mean marching into your Manager's office with a list. Sometime it's just something that you should use in your next performance review
"Expense challenge" is corporate bullshittery at its finest. Ask them how much it would cost to replace you. And technically, as your boss's position is currently vacant, you report to the CFO by default, and worth politely pointing that out to them and that these conversations then are to be expected with your direct reports.
1-to-1 meetings become important here - if your manager leaves then you should press very strongly that their 1-to-1 responsibilities to you are assumed by their direct boss rather than just quietly dropped...