I would suggest using your old domain, unless you are changing your complete, brand identity into a new entity.
1. Your domain name is a part of a hierarchy that Google reads, meaning that if you create a new domain, it will lose that ranking.
2. The new domain also means you will "tell" the search engines that you have a new website. The age of your website gives you relevancy according to Google and the older it is, the better you rank.
3. The activities that you have on your old website help with Google ranking and are indexed. This means you will either need to re-create these and build them back up with the new domain or may just lose some of the content and structure that you had.
Of course, there are several other factors that Google uses to help you rank, but these are all very important and take time to build.
If your website is being rebranded with completely different services, then it's worth making the change. Brand identity, quality and transparency are always first. If it is simply additions and changes, then it's best to keep the SEO value from the site you have. Continue to use what worked for you with the SEO and add into it as an improvement.
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