Recently, I was told by my manager that he wants to promote me from my current position as a Senior Engineer to Team Lead for my team when the company undergoes some reorganisation later in the year. I'm obviously very excited for this, but I'm also very nervous, not just because I've never been a team lead before.
For context, I joined the team relatively recently. Prior to my joining the company, the team was comprised of another senior and some junior engineers, all of whom are still on the team. Before I was hired the senior engineer was effectively the team lead, but our manager lost confidence tmin their abilities as a lead. Our manager has let me know what their concerns were that led to that loss of confidence, and I share them based on what I've been told. I've been (candidly) told the senior is on a PIP, with the expectation being that they would leave once they learned about the PIP, although from what I see day to day it looks as though they're trying their best to complete the PIP. Unfortunately they also appear to be trying to run interference with me i.e. requiring all changes have the entire teams agreement, trying to subtly disagree with my proposals, etc.
How should I approach this? Assuming that I do indeed get promoted, this will be my first time in any kind of official leadership position, and contextually you can see it probably won't be a soft landing even though I'll have my managers full support.
The overall advice boils down to frontloading the leadership responsibilities before you're in the "hot seat". Observe what the senior is doing vs. expected to do. Form your view on what the gaps could be and make a problem you jointly work on with your manager during your 1-1s. Go take up some responsibilities here and there, so you can get early feedback and have ample time to adjust your approach if things are suboptimal at the beginning.
Although it may feel tempting to let the senior keep doing what they're doing as the status quo until the PIP ends, I would lean on the side of finding ways to make the overall situation better and apply some of the solutions you and your manager came up with during the 1-1s.
The senior on a PIP is under a tremendous amount of stress, so it's normal to expect some combative behavior/aggression to come out as they are going through this. By implementing what you can do directly or through other people to make the project/environment better will like you mentioned, make it easier for yourself when you do take up the leadership position. Another point to keep in mind is that although the senior may be on their way out, they can make the transition reasonably smooth or a complete disaster depending on their relationship with you.