This creature's toughness cannot change. It would prevent any up or downds in the same way as "your life total cannto change"
If an effect or ability would reduce this creature's toughness to 0 or lower, it reduces to 1 instead. It would prevent the creature from dieing from having 0 toughness, but wouldn't prevent ups or downs.
The comparison you bring up with the first ability doesn't quite hold. This is because life total changes can only happen with discrete events, i.e. being dealt damage, paying life, or gaining life. On the other hand, toughness changes can happen with both discrete events (such as effects from Giant Growth) or continuous effects (such as Glorious Anthem).
Specifically, your wording doesn't properly define what happens with effects that set power and toughness to specific values. This is true for both discrete effects (such as "Target creature becomes a...with base power and toughness 1/1") and for continuous effects (such as "Creatures you control are 1/1...").
The second ability doesn't prevent creatures from getting their toughness lowered to 0 or less through static abilities (such as "Creatures you don't control get -1/-1"). As mentioned before, static abilities don't cause events to happen, and since replacement effects only deal with events, they can't interact with toughness changes that aren't caused by events.
Private Mod Note
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Rollback Post to RevisionRollBack
How to use card tags (please use them for everybody's sanity)
[c]Lightning Bolt[/c] -> Lightning Bolt
[c=Lightning Bolt]Apple Pie[/c] -> Apple Pie
Vowels-Only Format Minimum deck size: 60 Maximum number of identical cards: 4 Ban list: Cards whose English names begin with a consonant, Unglued and Unhinged cards, cards involving ante, Ancestral Recall
If an effect or ability would reduce this creature's toughness to 0 or lower, it reduces to 1 instead. It would prevent the creature from dieing from having 0 toughness, but wouldn't prevent ups or downs.
Specifically, your wording doesn't properly define what happens with effects that set power and toughness to specific values. This is true for both discrete effects (such as "Target creature becomes a...with base power and toughness 1/1") and for continuous effects (such as "Creatures you control are 1/1...").
The second ability doesn't prevent creatures from getting their toughness lowered to 0 or less through static abilities (such as "Creatures you don't control get -1/-1"). As mentioned before, static abilities don't cause events to happen, and since replacement effects only deal with events, they can't interact with toughness changes that aren't caused by events.
[c]Lightning Bolt[/c] -> Lightning Bolt
[c=Lightning Bolt]Apple Pie[/c] -> Apple Pie
Vowels-Only Format
Minimum deck size: 60
Maximum number of identical cards: 4
Ban list: Cards whose English names begin with a consonant, Unglued and Unhinged cards, cards involving ante, Ancestral Recall