Transferring Historical XP
This pool of HXP is locked to the variant on which it was spent, but can be assigned as the player desires to all duplicates of that variant in their possession.
For example: Under the previous system the player spent a combined total of 50,000 experience points (XP and GXP) on your SDR-5D. With the release of the new Skill Tree, all SDR-5Ds in your MechLab will possess a shared pool of HXP equal to the amount of experience the player previously spent on that variant. From the above example, the HXP pool for all SDR-5D variants in their MechLab would thus be 50,000 HXP. Historical XP can then be transferred to any SDR-5D variants in their collection. The player could transfer 25,000 Historical XP into 'Mech XP for one 5D, and 25,000 HXP into 'Mech XP for the other.
There is no cost associated with transferring HXP into 'Mech XP. The HXP transfers are permanent. Once the player transfer HXP to a specific 'Mech the player cannot reverse that transfer.
In the example below, the player owns three Warhammer 'Black Widow' Hero 'Mechs. Under the original Skill system these 'Mechs were treated as duplicates, and thus shared the same Skill status and 'Mech XP values.
Under the new Skill Tree, the player is able to set distinct Skills for all of the 'Mechs in their possession. These three variants are now considered distinct.
If a 'Mech has a pool of HXP from which to draw it will be marked with a new HXP icon in its Select 'Mech portrait, as seen in the example below.
When viewing the Skill Tree for a 'Mech possessing an amount of HXP, the 'Mech Enhancements window will contain a section dedicated to allocating any amount of that HXP onto the 'Mech you're currently viewing. In the example below the player is intending to transfer 20,000 HXP from their pool of 43,218 total HXP for the WHM-BW variant.
Adapted from the May-2017 patch notes.