Use form __init__ value

Try this

from django import forms

class FishTransferForm(forms.ModelForm):
    alias_name = forms.CharField(disabled=True, label='Fish to Transfer')
    current_sponsor = forms.CharField(disabled=True, label='Current Sponsor', required=False)
    new_sponsor = forms.ChoiceField(label='New Sponsor', required=True)

    class Meta:
        model = CustomUser
        fields = ('alias_name', 'new_sponsor')

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

        if self.instance.sponsor:
            current_sponsor = CustomUser.objects.get(id=self.instance.sponsor)
            self.fields['current_sponsor'].initial = current_sponsor.alias_name

            new_sponsors = CustomUser.objects.filter(sponsor=self.instance.sponsor, status__gte=3)
            new_sponsor_choices = [(user.id, user.alias_name) for user in new_sponsors]
            self.fields['new_sponsor'].choices = new_sponsor_choices