First you have to get your new seating and then you can assign the new seating straight on the planes you want without stopping the schedule … it will be then visible as the new seating in the Fleetmenu but it will be visibly booked only after 3 days …
Buy on credit, cash or leasing, that doesn't matter for the seat configuration of your aircraft.. But may be you have not enough money to purchase the new seats ?
you cannot edit/change your cabin configuration. You have to create a new (extra) cabin configuration, and then apply that configuration to your plane.
Under the 'Service & Marketing' dropdown, select 'Cabin Configurations'. On the configurations page, there's an area at the bottom to create a new config, just select the type of plane and create a name for it. Then you can choose what sort of seats you want, but be aware that better seats = fewer seats, and the number of flight attendants for each class of service.
Also keep in mind that we're getting a new cabin editor tool soon (as per this blog/post) so you'll need to re-create a new config once the new tool comes into effect. It does cost you to replace a config, so you may want to hold off until this happens.
As for a forum signature, at the top right of the forums is your handle with a dropdown box, open your profile and edit it, the forum signature box is one of the sidebar tabs. The instructions for the server-generated images can be found here.
So go to fleet mangement under operations and tick the box on the left side of the aircraft then an action choice will come at the bottom. Scroll down and select the cabins configuration you want and tick it, you might want a transfer as well, assign crew if it is not done automatically and what fleet you want your aircraft in. CLICK EXECUTE!!!