No, if a voucher is sold when the voucher type settings have "Partial redemptions" disabled, if you then change the voucher type settings to have "Partial redemptions" enabled. This won't update already sold vouchers.
For example, see scenario below:
This voucher type has "Partial redemptions" disabled...
This voucher was purchased. Therefore, in the voucher code page, you can see it states "Partially redeemable" is "No"...
If I then head back to the voucher type settings page linked to the voucher code and change this to allow partial redemptions by toggling on "Partial redemptions" and "Save" to save changes...
This does not have any impact on the already sold voucher, it still will not allow partial redemptions...
Therefore, to clarify, if you sell a voucher when the voucher type settings page has "partial redemptions" toggled off. If you later, toggle this on this will only be relevant for new vouchers purchased it won't update vouchers already sold, these will still remain as not allow partial redemptions.