Has there ever been any information about the exact length to be re-fitted from headspan to portal? I seem to remember an idea that it was only certain complex areas such as running junctions, but if it is to be the entire length from Paddington to Airport Jn is there still a lot to do, and is it still (AIUI) covered by Crossrail “on network funding”?
Most of the larger-spanned tracks are under the square-boomed, hinge-fixed "Vierendeel" portals installed as part of the Heathrow Express electrification (i.e. around Paddington); some of them are spanwire portals, so I suspect that they'll get independent registration from the same booms, done, as you suggest, on an "as-and-when-network-funding-becomes-available" basis.
From what I've seen so far, renewals & replacements with mechanically independent arrangements have been concentrated around station areas and complex junctions (such as the eastern approach to Ealing Broadway, the crossovers at the country end of Southall Dn Yard, and the Hayes & Harlington area IIRC).
Due to it being listed, Hanwell (& Elthorne for the traditionalists among us) might remain a headspan location, especially as it has cylindrical hollow masts rather than universal columns ('H' sections). There are also a few headspans still extant west of Acton Main Line - my penn'orth's going on them being replaced by portals that also span two tracks or so in Acton Yard. Some were already repalced as part of the dive-under works a few years back.
Hopefully the Poplar chord will get wired...one of these days...