Montporte Dungeon Campaign. I could think of a couple of different ways to do it this. One way would be to convert experience points directly. However, because Blood & Treasure has three different experience point tables compared to one for D&D 5e, characters could potentially gain or lose levels based on a meta-game activity.
Instead, I decided to convert each character based on their current level and how far they were from making it to the next level (stated as a percentage). I then placed them the same distance away on the D&D 5e experience table. So if your character is 5th level and has 34% of the experience needed for 6th level in Blood & Treasure, you will still be in exactly the same place in D&D 5e.
The upside is that all characters are still in exactly the same position on the experience table that they were in before. The downside is that character on the "steepest" experience point table (e.g. wizards, sorcerers and warlocks) lost experience points relative to those on the least steep table (e.g. thieves).