Jessey is right - always change the exercise regime before the feeding regime, so as not to upset her digestive system and cause azoturia or laminitis in particular. if and when you do so, change her feeding regime over a period of at least 7-10 days - 2 weeks.
her diet does need reassessing as you thought, and although it provided plenty of energy considering that you are feeding a conditioning feed with chaff, and a fair amount of it, i would suggest that what your horse is lacking most of is fitness!
you will find that, if you steadily increase her workload and work on short bursts of fast work (which fittens) and slow work (if she is needs to lose weight at all) she should become more energetic as her fitness improves and she gets used to her new exercise regime.
5-6 days a week (doing light work i presume) is not really taxing, but is abrupt from 2-3 days per week, so go easy for the first few weeks, graduaklly building up the amount of fast/long work, etc.
you can always contact a qualified nutritionist (try Dengie, Spillers or Baileys for example) for no fee if you remain uncertain, for more details.
for now though, i would increase the amount of feed but change the actual brand to a leisure cube or mix (mixes tend to contain more starch, sugar and less fibre than cubes, so if she is a good doer/prone to laminitis then a cube is much preferable and healthier), for example Baileys Economy Cubes or Spillers High Fibre Cubes.
This is because the 2 small scoops may provide her with enough energy (though it isn't!
) but is deficient in essential nutrients for your hros eto remain healthy and active.
if your horse weighs (healthily) about 400-500kg then she should be receiving 8-10 kg each day of food, and approx. 2kg of this should be hard feed in order for her to receive an essential balanced diet alongside plenty of forage.
an alternative (if she is a good doer or prone to laminitis especially) is to feed a forage-based 'hard' feed for energy, with a feed balancer (such as Baileys Lo-Cal, Topspec Balancer or Blue Chip original/Lami-Light) or vitamin and mineral supplement (such as Dengie Optinum or Global Herbs Gloabvite) to provide a balanced diet and all she needs to maintain the best of health and fitness. an advantage of this compared to a balanced cube/mix feed is that you can alter the aounts of the fibre-feed without upsetting the nutrient balance, according to your horse's workload, weight, the time of year, etc.
if she sweats a lot then you will need to add a tablespoon of espom Salts to her water (with another normal bucket of water) or a damp feed, or you can use a ready-made electolytes formula (such as the Sport Likit availiable
ad-lib).
if this sounds confusing (duh!), i'm only a PM away to re-explain! :-D
good luck!