Hi Ruth,
Thank you a lot for your input. I think our changes to brand and website will not be that significant as we do not change anything regarding our brand - it will remain the same. Same for the design and content.
I like the idea of collecting email addresses, but I am not sure what to tell them when we switch domains - as there will be nothing new, apart from the domain URL
The major issue what was bothering us was the fact that our main domain was country specific (.at) and we are an internationally operating company. That is why we planned to switch the main domain to a .com.
I will definitely follow your suggestions about some PR before the domain switch and get SEs to crawl the site before the switch.