If I have Klefki with "Mischievous Lock" Ability active and switch or retreat it to an Iron Valiant ex with "Tachyon Bits" Ability, can I use Tachyon Bits?
Yes, when Iron Valiant ex switches in Klefki is no longer there, so Mischievous Lock does not prevent the use of Tachyon Bits.
Source: TPCi Rules Team (2023-11-09)

Relevant cards

Note: All applicable cards may not be listed below.
SV1_96
SV1_96
SV4_89
SV4_89