Yes, your remaining amount (calculated on prodata basis) will be adjusted.
Let's take an example, if you have purchased 1 year 1 device plan for 2000 rs
and after 3 months, if you switch to lifetime plan, in that case
2000/12*3 = 500, you have used 500 rs
remaining 1500 will be adjusted in lifetime plan
Say, lifetime plan costs 8000 rs - 1500 (adjustment) = 6500, you just need to pay.
Kindly note, GST will be applicable on all plans, if you pay in INR or Indian resident.