You choose the target for Soul Seizer's triggered ability when that ability triggers and goes on the stack. You choose whether or not to transform it when that ability resolves. If the creature is an illegal target by that time, the ability is countered and none of its effects happen. You can't have Soul Seizer transform.

If Soul Seizer leaves the battlefield in response to its triggered ability, you won't be able to transform it and it won't return to the battlefield or become attached to the target.

If Soul Seizer has any +1/+1 counters on it when it transforms, they remain on Ghastly Haunting. They won't affect the power or toughness of the enchanted creature, however.

If Soul Seizer deals combat damage to more than one player at the same time, perhaps because some of its combat damage was redirected, its triggered ability will trigger once for each of those players. Say you transform Soul Seizer into Ghastly Haunting for the first such ability to resolve. For the next ability, you may choose to transform it back into Soul Seizer. If you do, it will be a creature again and won't attach to anything; you aren't able to do anything between choosing whether to transform it and unattaching it.