I think the answer to this depends on so many things: where you live (far north of Scotland I see, so very different to the south of England); the quality of the grass you have (there's not much nutrition in grass this late in the year); the condition of your sheep earlier in the year as well as now; local soil type (which could create some nutritional deficiencies); your own experience; the weather at the time; your gut feeling ie by looking at them to judge general condition - this is ongoing; age of the animals.
I don't think it's a good move to actually stop the hard feed at this time of year unless you've been overfeeding them. Next summer when they are fit and the grass is full of nutrients, then is the time they need no extra feeding. From now on in, as they are being bred this year, they should have a small sprinkling of something such as 'tup and lamb' coarse mix. What they definitely don't need, at any point, is high protein ewe pellets, which are completely inappropriate for primitives - way too much protein. What are you giving them currently? My Hebs, Shetlands and Soay have never had ewe pencils or nuts and don't need them.
A further point is that once the tups go in, they (the tups) shouldn't have any hard feed at all unless the snow is on the ground, or hard frost. it can be difficult to manage feeding ewes but keeping the tup away - will make him bad tempered !
Because we still have plentiful grass and they are not otherwise hungry, we haven't started the breeding ewes with even hay yet (our ancient toothless biddies are a different matter - they need nutritional support all year round, although they've not been interested this year because the weather and grass have been great). I would however offer hay in preference to cereal based feed. We keep the Tup and Lamb for the colder months, but once started we wouldn't withdraw it completely. We are in the south of Scotland with good grass (for this location, but poor compared to the lush pastures of Cheshire) but a poor climate - wet, cold, windy and snowy, so the sheep need good nutrition to cope with those conditions.
What is bothering me is the drastic change of condition of these sheep since you got them, so it's not as if you are starting out with fully healthy ewes. My feeling is that to suddenly completely stop the hard feed they have needed to build them up would be adding in an extra unnecessary stressor. Feeding once a week doesn't help the specialist gut flora to establish well, hence my suggestion to feed a tiny sprinkle each day.