Well, existig HSMP holder can very well apply for extension under Tier-1 (general), but going by what's written in the HO document, you are not affetced, if your previous grant of leave is under Tier-1 (General), so for HSMP holders that's not the case, as their leave is under HSMP, not Tier-1.guys,
Im being quite optimistic here, I dont think there is any need for concern for HSMP holders.
If you read the guidance section or on the UKBA site its clear that HSMP holders at the time of expiry of their visa will seek "extension" , so please do not confuse it with a new application.
But this is just me, Im sure the confirmation will come from the HO soon enough.
Pasting below what's there on the UKBA site under "Applying for extension section"
Can you apply to extend your stay?
You can apply to extend your stay if you are:
* already in the United Kingdom under the highly skilled worker category and want to extend your permission to stay within your existing category; or
* already in the United Kingdom under the Highly Skilled Migrant Programme, want to extend your permission to stay and are eligible to switch into the highly skilled worker category.
If you have not previously been given permission to stay in either of these categories, you should read the section on initial applications.
I sincerely hope, that this rule does not apply to existing HSMP holders, and that should be the case, if HO wants to play it fair, but as we all know the history......