I didn't say it was cheap though it is a straight forward task -Not a particularly difficult one at all. The issue is, as you say the logistics and timing of the roof replacement Removal would take at least a full summer and replacement would take a longer time period. For these reasons I fully expect the existing roof to be dismantled and a new one constructed - even then the logistics are quite frightening:
[1] Summer 1:
- Remove Stand [A] Roof.
[2] Season 1:
- Build stand [A] roof support towers and stand A extension.
- Assemble new roof sections for stand [a] next to ground.
[3] Summer 2:
- Assemble new stand [A] roof over stadium.
- Begin completion of stand [A] extension.
- Remove Stand Roof.
[4] Season 2:
- Complete and open stand [A] extension.
- Build stand roof support towers and stand extension.
- Assemble new roof sections for stand next to ground.
[5] Summer 3:
- Assemble new stand roof over stadium.
- Begin completion of stand extension.
- Remove Side Stand [C] roof
[6] Season 3:
- Complete and open stand extension.
- Build Stand [C] roof support towers.
- Assemble new roof sections for stand [C] next to ground.
[7] Summer 4:
- Assemble new stand [C] roof over stadium.
- Remove Side Stand [D] roof
[8] Season 4:
- Build Stand [D] roof support towers.
- Assemble new roof sections for stand [D] next to ground.
[7] Summer 5:
- Assemble new stand [D] roof over stadium.
Then at some point you would probably want to remove redundant masts form the Spirals.
As can be seen the ground will be a building site for around 4.5 years due to the logistics of the thing. The building of a new Stadium from the ground up is appealing though probably more expensive. Of course there is the alternative of becoming tenants elsewhere for a season or two (though I doubt that would be popular).