We have done the same no issues at all. I messaged Kendamil and they said you can easily swap between these 2 with no issues.
I bounced between classic and organic but have succumbed to buying a tub of stage two this week as has no other option! Places like Waitrose always seem to have stock (that’s where I’ve always found it when desperate!) and my local coop does but they seem to gate keep it in the back and not on the shelf!
We started on the normal one then switched to organic and now back to the normal one and we had no issues at all