I currently have an annual season ticket from Bedford to Kings Cross
Thameslink. I'm starting a new job soon that is based at St
Katherine's Dock, which means I need to get to Tower Hill each day.

I'm not sure which is my cheapest way to get to Tower Hill.

I was thinking about upgrading the season ticket to Blackfriars, or
City Thameslink and walking from there. Or maybe adding the travelcard
to the season, though I'm not sure whether I have to buy a Z1-6 or
whether I can buy just a Z1-2.

I'm wondering the difference in prices for Thameslink annual season
tickets from Bedford to Farringdon, City Thameslink and Blackfriars.
And, also how Thameslink upgrade a ticket. If I added a travelcard, or
wanted to extend the season ticket I have to Blackfriars, how do they
work out the cost? Do they add the extra costs on to season, charge me
the extra and give me a new ticket? Or, do they work out the new cost
of the newly priced ticket based on current price of the ticket and
charge me that?

Can anyone help?

Ian