in General any lvl change while they are active is bad, even if u go to a higher lvl and they stay.. they will stay on same lvl they was until u zone.
In Normal solo Mission its not much difference if u lvl up, that 1 lvl difference doesnt make them unplayable.
But in team or League, if u was 27 and suddenly leader is a lvl 50, the pets stay on 27 until u respawn em or zone. And lvl 27 pets when u are sk'ed to 49 are worthless even in Bodyguard mode.