No you should not be tied in unless you take free hardware as part of the upgrade - see item 6.4 from karoo broadband terms and conditions
http://www.k-c.co.uk/docs/legal-docs/karoo-terms.pdf6,4 . If You upgrade or downgrade Your Service, Your new Service will not normally be subject to a
further Minimum Period. However, if You take up an offer of free hardware as part of an upgrade,
Your new Service may be subject to a further Minimum Period. Any changes to the Minimum Period
for Your service will be clearly explained to You at the time You request the change in service.