slwatson
Hello and a belated welcome to the Forum.
Sorry, I don't know what kinds of birds you have, or how long you've had them.
However, as a general rule for most Finches.
No, I would not remove their seeds for a whole day. Finches are primarily seed eaters, and need to have access to seeds to survive (unless they are on a pellet diet). Trying to starve them into accepting a new diet, is NOT the way to go.
First of all, when you mentioned 'soft foods', are we talking about egg food, vegetables, etc. ?
If that is the case, and your birds haven't been fed these things before, then they probably haven't got a clue as to what to do with this 'strange' stuff.
My recommendation, would be to start slow and in small amounts.
First of all, egg food can be made quite irresistible by a few small steps.
Hard boil egg (s)
Peel, and save the shells (for later use, dried and crumbled and offered as birdie calcium rich potato chips).
Run egg through a garlic press (or mash with fork and fluff up)
Crumble a sizable sprig of spray millet into the mixture and toss together to mix.
You can also add a small amount of pulverized store bought egg food to the mixture, and also some pulverized freeze dried mealworms.
However, if you don't happen to have the two above mentioned, just the spray millet will do.
This mixture, can be made the evening before and fridged for a couple of days or served fresh .
It has been my experience, that most birds will give it a try, and never look back. While trying this new delish food, I would withhold other spray millet treats, but continue to offer regular seed mix.
As to vegetables and greens.
Not too sure what you have been offering your guys, but here are a few pointers.
As a first , I would offer a very simple romain lettuce leaf , stuck through the cage bars.
In many cases, birds may have been introduced to romaine by some well meaning if somewhat misguided soul, who believed that lettuce is the only veggie that birds can eat.
I would offer it, but don't push it, let them figure it out.
Next will come the irresistible half slice of an English cucumber, again, stuck through the cage bars, or clipped into place, cut side facing upward.
After they figure this one out (which most birds usually do ) I'd start offering small amounts of other items, to find out what they like best.
When it comes to choices, the sky is the limit.
Kale
Spinach
Carrot greens
Grated carrots
Beet greens
Red swiss chard
Parsley
Packaged baby spring greens
Broccoli
Corn on the cob
Dandilion greens
Mustard greens
Chopped apples
A half of a mandarin orange
Bananas
Melons
Strawberry
The main ingredient, is patience.
Good luck
~Elana~