The reading of the Legendary surge ability is as below.
The legendary surge ability allows the bearer to add the result of a d6 to the appropriate type of roll or check. A mythic bearer can use her surge die type in place of the d6. If she's bonded to the item, she can increase that die type by one step.
I understand that a mythic character can use their surge die in place of the legendary surge die but the last sentence is not clear.
Is it limiting the die to one die step up when using legendary(So the highest would be a d8), OR does it mean if the character's surge die is a d8 the surge would be one step higher and be a d10?