You can choose not to have Omni-Changeling enter as a copy of another creature. If you do, it will just be a 0/0 Shapeshifter with changeling, and unless another effect is increasing its toughness, it will be put into its owner's graveyard.

You can tap any untapped creature you control to convoke a spell, even one you haven't controlled continuously since the beginning of your most recent turn.

Any "enters" abilities of the copied creature will trigger when Omni-Changeling enters. Any "as this creature enters" or "this creature enters with" abilities of the copied creature will also work.

Changeling is a characteristic-defining ability. It functions in all zones, not only while a card that has it is on the battlefield.

If a creature you control has a mana ability with {T} in the cost, activating that ability while casting a spell with convoke will result in the creature being tapped before you pay the spell's costs. You won't be able to tap it again for convoke. Similarly, if you sacrifice a creature to activate a mana ability while casting a spell with convoke, that creature won't be on the battlefield when you pay the spell's costs, so you won't be able to tap it for convoke.

If an effect causes a creature with changeling to become a new creature type, it will be only that new creature type (unless the effect says "in addition" or similar). It will still have changeling; the effect making it all creature types will simply be overwritten.

If an effect causes a creature with changeling to lose all abilities, it will remain all creature types, even though it will no longer have changeling. This is because changeling applies before the effect that removes it.

If the copied creature has {X} in its mana cost, X is 0.

If the copied creature is a token, Omni-Changeling copies the original characteristics of that token as stated by the effect that created that token, with the listed exception.

If the copied creature is copying something else, then Omni-Changeling enters as whatever that creature copied, with the listed exception.

Omni-Changeling copies exactly what was printed on the original creature and nothing else, with the listed exception (unless that creature is copying something else or is a token; see below). It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, and so on.

Tapping an untapped creature that's attacking or blocking to convoke a spell won't cause that creature to stop attacking or blocking.

The subtype Shapeshifter that appears on the type line is mostly there to reinforce the flavor. A creature card with changeling is just as much a Kithkin, a Goblin, a Merfolk, and a Brushwagg as it is a Shapeshifter.

When calculating a spell's total cost, include any alternative costs, additional costs, or anything else that increases or reduces the cost to cast the spell. Convoke applies after the total cost is calculated. Convoke doesn't change a spell's mana cost or mana value.