This depends on the bearer your leased line is running on. A bearer essentially sets the limit of your bandwidth, so it's important to ask about the bearer before signing up for a leased line. For example, if you choose a 10Mbps leased line on a 100Mbps bearer, you should be able to contact your provider at any point in your contract and request a faster speed (for a higher monthly fee, of course). However, if you have a 100Mbps leased line on a 100Mbps bearer, you've hit your limit, and you'll need to wait until your contract expires before upgrading to a faster speed.