can you replace 86T with 88H?

86 and 88 are the load index, T and H are the speed rating. H is a higher speed rating than T (it's a weird code, Google helps), so you're good there.

Comparing apples and apples, 88 is a higher load index than 86, so that would be ok too. But, if you really meant the one size starts with a P and the other doesn't, these are not apples and apples.

The sizes that start with P are standardized according to the "P-metric" system overseen by the Tire and Rim Association. Sizes without P follow "Euro-metric" standards overseen by the European Tire and Rim Technical Organisation.

Both systems have the idea of a "load index", but it works completely differently in the two systems. More information here.

So, if you're comparing a P-metric load index 86 to a Euro-metric load index 88, you would really have to look both up and see how many pounds load capacity each one corresponds to for that tire size. (The same P-metric load index number means different load capacities for different tire sizes. Euro-metric load index numbers don't.)

Clear as mud?