Lithium ion batteries in place of lead acid

I I swapped out the 36 volt sealed lead acid batteries in my EV Global e bike with 36volt lithium ion batteries, can I keep using the built in charger used to charge the lead acid batteries?

YES/NO

You can use it but you will end up undercharged. Best to get the proper charger to take advantage of the max storage.

I I swapped out the 36 volt sealed lead acid batteries in my EV Global e bike with 36volt lithium ion batteries, can I keep using the built in charger used to charge the lead acid batteries?

YES/NO

You can use it but you will end up undercharged. Best to get the proper charger to take advantage of the max storage.

The only reason it would be under charged is if the charger at no load voltage is less than the 3.8V/cell needed to get to full charge.
If the a 36V lead acid should float charge to 2.3V/Pb-cell x 18 = 41.4V
So a liIronPOOOO pack charged would be 3.8 x 12 = 45.6V
So if you use 11 cells of LiIronPOOOO then the charged pack voltage is 3.8 x 11 = 41.8V
Thats 0.4 less for the whole pack, it depends on the BMS setting for full voltage.

Now if you have a BMS on your pack that cuts off charging when they are full at 3.795V your fine. These decimal places just show that is a bees little toe of a difference.
So it comes down to what your Lead Acid Charger does.

But if you don't have a multimeter and feel like you know what to check... best to get someone to measure your charger characteristics.
Or by a charger for $100. (I hope less)

Have you got a picture of your charger? The model and ratings?