Once a membership (subscription) is active/published, the price can't be changed. You can remove it (unpublish) from the join page, but members who are already on that price would continue to be billed at that price.

You can always add new, memberships with different price points, but those would be available only to new sign ups. Active members would have to cancel and rejoin to get the new price. 

Membership prices can be published or unpublished at any time.