makin living space out of attic?
What is your cost to benefit ratio? Will it increase your property value enough to justify demolishing then re constructing the entire roof system to add just a few inches?
In most cases: Hardly.
You would do much better to add a full second stroy than tinker with adding a few extra inches of 'head room'.
Will your codes allow it?
Minimum modern headroom requirements are at least 6'8" and more like 7'+ in most instances.
Adding a few inches for headroom may not even be legal where you live.
Can you modify your attic without making major structural changes?
In most cases NO.
Once you begin to modify an existing attic you need to conform to modern codes. This could involve increasing the depth of existing floor joists which almost always requires the replacement of any steps or stairs leading to the attic.
Attic conversions are rarely easy and even more rarely cheap.
THINK before you act and get your local building code officer's input before you do anything else.