I think you are getting two things confused (if you will forgive my saying so).
The promise of your brands at present is surely about how they make people feel? The fact that you are adding the convenience factor of making them all available under one roof is great, but is not an enhancement of the individual brand promises; it is part of a separate accessability message that will be attached to the new ‘umbrella’ brand.
If you replace the emotional promise (we’ll make you look and feel great) with a convenience promise (all under one roof), you have not aggregated the two into something bigger, you have replaced one with the other – and I would suggest substituted a weaker for a stronger.
The ‘all under one roof’ message is great as part of a strapline, but that is where it belongs. It should not affect your choice of ‘umbrella’ name which should seek to feed off the common emotional thread running through the existing brands. It is this gut attraction to the brands that will continue to drive sales and brand value; the convenience factor should boost the level of sales but is subordinate in terms of the overall role it plays.