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If it is of any interest/help, I discovered my 647 (& 647A) EHT transformer issue about 20 years ago. I had no idea why it failed only that it did mainly by using increasingly excessive amounts of current to produce increasingly less display brightness and significant heat. Clearly Tek were aware that this failure mode was possible/likely? - the EHT drive circuit has its own fuse.

To get some use from the otherwise excellent 647A - I went into an ebay parts only 647 buying frenzy until I ended up with 6 or so, from which I would extract the EHT transformers and swap them from dead to living scopes as needed, tedious and barely worthwhile in gaining between zero and 20 minutes useful operation.

So that solution having failed I had a UK based company wind me another - I have the winding specs - so they assured me they could start from scratch. Back it comes with a vague hint of future operational happiness - not so

The only success, albeit short lived, was to bake the transformer. We have an 'always on' range (an Aga) and the plate warming oven sits at about 70C. Increasing the bake time increased working time at the approximate ratio of 1 bake day to 10 mins useful operation on a non linear and only loosly reliably basis. 6 months gave me 40 minutes use. This seems to be close to the max - Even after 2 years baking 60 minutes was the maximum.

So from my limited experience once the transformer knows its life is coming towards the end, rejuvenation/resuscitation prolongs but does not renew.