Backing out of a verbal postdoctoral job agreement with my PhD advisor I recently finished my Ph.D. and had a strong interest in working with a particular professor as a postdoctoral fellow upon finishing. This professor happened to be the examiner of my thesis, so I could not contact him until I completed the thesis defence.
Before the thesis defence, my Ph.D. advisor offered me to stay as a postdoc for 2 years. I agreed since it was the best opportunity available at the time. The agreement is verbal, and I am now awaiting the official written job offer.
Upon finishing my defence, the examiner contacted me and showed a strong interest in hiring me (as soon as possible). It seems that we both think that we are an excellent match. He is also an excellent professor that I think will help me getting ahead in my career. It seems to be a much better opportunity than what I have now.
The only problem is the promise I made to my Ph.D. advisor, who strongly wants me to stay with him. I have a few questions about this situation:
1- What would be the best approach to handle this?
2- How bad will I look of I back out on the verbal agreement?
3- Could this cause damage to my career or burn bridges with my Ph.D. advisor?
4- Considering that I will need recommendation letters from my Ph.D. advisor in the future, how likely will this affect my ability to obtain these letters?
5- A solution I am considering is staying temporarily (6 months) with my Ph.D. advisor so that I am not fully backing out on my word. What do you think?
TLDR: Many of your questions depend on details of the field and your personal relationship with both professors involved. The best you can do is handle this like any other professional opportunity by putting yourself in your advisor's position and thinking of ways to turn things positive for all parties. Assuming reasonable people on both sides, clear and honest communication on your end has a good chance of producing a satisfactory outcome. Your idea in questions 5 should imho only be done when the 6 months are clearly communicated as transition period to train your successor.
The answers to your questions depend strongly on details of your involvement in the lab in which you did your PhD and the relationship between your advisor and the other professor. A few (but by no means all) things to consider are:

Do ongoing projects depend on you (e.g. because you're the only one
who knows how to handle lab equipment, simulation code you wrote or
similar)?
Are you currently supervising undergrads in their research?
Does funding for the lab depend on you personally working there (e.g. if you acquired grants that go whereever you go)?
Do your advisor and the other professor get along professionally or would you be 'switching allegiances'? The fact that they both served on your defence committe suggests to me that they would get along, but this may depend strongly on field and country / academic culture. If they get along, maybe they can cooperate via you and publish a few papers together.
How much trouble did your advisor go to in order to secure funding for you?

Answering these and similar questions are a means of determining, how badly your walking back on your promise would set your advisor back. Once you have established this, you have a much better perspective of the gravity of the situation and may find means of making it less hard on your advisor (or realize that it is not that big a deal after all - in many cases, I've been very cautions and even nervous about situations like this just to get responses like &quot;That offer sounds great, how about a 6 month transition period to hand over ongoing projects, then go for it!&quot;).
There are also several upsides for your advisor: Switching positions after your PhD might be generally expected (depending on field), and is generally benefitial to your careers as you will be exposed to different ideas and a whole new network of possible collaborators. Having students that rank high in academia is always good for the advisor. Also a more in-depth cooperation with the other professor might benefit all parties.
For the moral perspective: What's being done to postdocs contract-wise in academia would be unthinkable for most other employees almost everywhere. Strings of 1 year contract, having to acquire own funding, you're expected to be mobile on a global scale, relatively little money given the value of your work force in the industry etc. That it would in my opinion be unreasonable to condemn your for having accepted the only job offer you had on hand at that time. However, I cannot tell whether your advisor is aware of this. Many are not or deem it beneath them to think in such frivolous terms as quality of life of worker's rights, because &quot;we're scientists&quot;, and they're part right. Since a career in science is always somewhat of a gamble (unless you get a Nobel prize out of your PhD), it takes a lot of idealism to stick with it in the long term.
As for your suggestion to stay 6 months and then move on: I would only recommend that if these 6 months are clearly communicated as a transition period (to hand over ongoing projects, train the next person on equipment and code etc). If you do not tell your advisor that you will move on after 6 months, you've technically held your promise to sign the postdoc contract but violated the spirit of your promise anyways, which may just create more harsh feelings.
At any rate, feel good about your job perspectives (since at least two independent parties are trying to secure your services) and congratulations on your newly acquired title.