Over a year ago, we updated the email editor when using it from the main people page or a person's profile. We ironed out some bugs over time, but now that it's been stable for so long, it was time to start using that same emailer everywhere else.
When you send an email anywhere from a plan or from within the Matrix, you'll now see the new email editor. In fact, you should see that email editor no matter where you send an email from within Services.
When you send an email from a Plan, you are sending a Scheduling email, therefore the only email templates you will have access to are Scheduling templates, unless you choose to send a Signup Sheet email, in which case you'll have access to Signup Sheet templates instead.