My advice and that of others is that if you are going to replace the batteries, you should replace all of them at the same time.
The low voltage batteries will certainly drain power from the higher voltage ones and in the process they will all fail at some point.
Any series/parallel battery arrangement at least needs to have e.g. in a 24v system of paralled banks of 2 x 12v batteries in series all the mid points of those series units connected together to try to balance the voltages across each battery and best to also install a battery balancer that will ensure that mid point stays at half whatever the charging voltage is.
We have this arrangement but with 4 x 6v batteries in series and 3 banks of those paralleled. The batteries are AGM and they have been in the boat for 5 years this April with no noticeable loss of capacity - yet.