Hi Dsouza_28,
When a score reaches the sales-ready threshold, a plug-in automatically sets the sales-ready flag on the lead to True. If a lead is scored by multiple models, the flag is set as soon as any model reaches the threshold. This triggers a workflow called "Lead to Opportunity Stage Transition", which automatically moves the lead to the sales-acceptance stage of the lead lifecycle.
https://learn.microsoft.com/en-us/dynamics365/marketing/score-manage-leads#establish-grades-and-the-sales-ready-score
Based on the documentation, it seems that no OOB process automatically qualify lead.
In your scenario, are salespeople manually qualified or is there some other customized automated process?
1. What happens to the Leads that were already Qualified as Sales ready?
For lead itself, there is no impact.
And BPF is still in the sales acceptance stage, although the score does not reach the condition, but it will not return to other stages and the lead status will not change.

2. Will these leads get re-calculated and can get qualified again?
Yes, leads will get re-calculated, but will not be qualified again unless you have other customizations.
3. What happens as result when a lead model is deleted?
We can't delete scoring model, but you can stop it, it will be changed to draft status.

After stopping it, scores will missing from all leads, but sales acceptance stage and lead status can't be changed.
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Note:
Everything above is answered without any other customization.