length of sidings+length of platforms is an issue.
Had the platforms/yard lines already been long enough to handle 30 coaches then increasing to 26 coach wouldn't be an issue, in that case only the production capacity of coaches would be a limiting factor, this isn't the case sadly and in many cases trains barely fit on Pf when 24 coach long.
there are certain PF's though which have heard have been extended, so the process may not take very long, but instantly it won't happen.
it...
more... is a much needed upgrade though, hope it takes place soon.