If you are changing your domain name, you need to consider two things when deciding whether to continue paying for your old domain.
1. How often do links to your site appear around the web, in emails, in guest blog posts, etc.?
If the URL to your old site has be widely distributed, and those links are still relevant (meaning people still might click on them from time to time), then it is very important that you do a 301 redirect and continue paying for that domain as long as you think those links remain relevant/seen. There is nothing worse for your practice than having a potential client find a 404 Page Not Found error when they wanted to access your site.
2. How long have you owned your domain? How many people have visited that domain over the years? Is your site ranking?
If you've had your domain for over a year and many people have visited and spent time reading your content, then you likely have strong domain authority which you'll want to maintain for SEO purposes. When a 301 redirect is put into effect, the ranking power from one domain is transferred to another domain. It is recommended that you continue paying for your old domain for at least one year after the 301 redirect is completed.
If you have further questions about 301 redirects, please contact your Project Manager or submit a support request.