I believe you could run it anyway for no XP or rewards.
Or Ouroboros it!
K5K - The Killbot 5000
A Spanner In The Works Part One, ArcID: 336662, A Spanner In The Works Part Two, ArcID: 336665, Enter Japes, ArcID: 96001
In The Darkness Creeping, ArcID: 347709, When Dimensions Collide, ArcID: 412416.
I still have this happen from time to time, but if I start in on an arc and eventually outlevel it, the contact still gives me the missions for the remainder of the arc, but they are capped at the top-most level for the arc. I.e., I have a high 40's hero that just completed an arc from Gordon Stacy whose arc missions are well below those levels. The enemies in the missions were well below my level.
However, if you don't start the arc before you outlevel it, it's gone from that contact and you need to Ouro it.
Main Character: Ice/Storm/Ice Controller (Justice, 1340 badges)
In fact, if the arc has been started, you will eventually have to run it for no XP (or drop each mission using mission drop) to clear out space for other arcs, since you have a limited number of arcs open at any given time.
AFAIK, doing the Ouro version of the arc will not clear it from your limit of open arcs.
Altoholic - but a Blaster at Heart!
Originally Posted by SpyralPegacyon
"You gave us a world where we could fly. I can't thank you enough for that."
[ QUOTE ]
Let's say an arc begins at level 1.
Some missions are intended for level 5.
I gain level 10, before I get to these missions. Now I've outgrown them.
Can I skip them and pick up the arc from a later mission or is it lost to me and I should Ouroboros it?
[/ QUOTE ]
If you're in the arc ( the book icon appears in the contact window) then you will have to complete it to clear it. The arcs do have a level max ( the same as the contact's max level). So if you were doing an arc in the 5-10 range, didn't finish it, and came back to the contact at level 30, you'd still have to complete the missions and they would be set at level 10.
If you use Ouroboros, you will not complete your unfinished arc, but will start a new flashback from the beginning.
The good news is that even though you get no XP or rewards from very old arcs, you will get the merits on arc completion.
Kyle Al'Mordu - Lvl 50 MA/SR Scrap
Fusion Force!
TRIUMPH
If you have already started the arc (doing a single mission is usually enough to open an arc villainside, you may need to do several heroside before the arc begins) you can come back and finish it at any time, but the enemies will be set for the maximum level for the arc, which means they will likely be worth no XP for you.
If you outleveled the arc before starting it there is no way other than Ouroboros to experience that arc.
Contacts with multiple arcs can be outleveled even if you started their first arc previously. If you outlevel the contact before starting their second arc you don't get that arc. Again you have to go to Ouroboros to do the arc.
[ QUOTE ]
In fact, if the arc has been started, you will eventually have to run it for no XP (or drop each mission using mission drop) to clear out space for other arcs, since you have a limited number of arcs open at any given time.
AFAIK, doing the Ouro version of the arc will not clear it from your limit of open arcs.
[/ QUOTE ]
Note: Story Arcs (Blue Bookmark) are limited to two.
Mini-Arcs (Green Bookmarks) are not limited at all, though there have been bugs with some of them (RWZ) limiting both Story Arc and Mini-Arc receipt.
Oh! You can turn off XP now. Under Menu - Options - General Tab - Miscellaneous - Disable Earning XP. So, you'd be able to slow down your progress on an arc if you really like where it's going but fear you'll out-level it.
Pst, while you're there you may as well turn off those pesky in-game ads for real-life products. That's just my pet peeve. 
K5K - The Killbot 5000
A Spanner In The Works Part One, ArcID: 336662, A Spanner In The Works Part Two, ArcID: 336665, Enter Japes, ArcID: 96001
In The Darkness Creeping, ArcID: 347709, When Dimensions Collide, ArcID: 412416.
[ QUOTE ]
Oh! You can turn off XP now. Under Menu - Options - General Tab - Miscellaneous - Disable Earning XP. So, you'd be able to slow down your progress on an arc if you really like where it's going but fear you'll out-level it.
Pst, while you're there you may as well turn off those pesky in-game ads for real-life products. That's just my pet peeve.
[/ QUOTE ]
You do realize there hasn't been a new ad in game for over 2 years now?
I turned them off on day one of the system being introduced, but have been keeping track of any discussion on the forums and at HeroCon about it, didn't seem like 2 years, but hey. *shrugs*
The in-game ad system is the Rikti to my Vanguard! They are among us! I will be vigilant.
K5K - The Killbot 5000
A Spanner In The Works Part One, ArcID: 336662, A Spanner In The Works Part Two, ArcID: 336665, Enter Japes, ArcID: 96001
In The Darkness Creeping, ArcID: 347709, When Dimensions Collide, ArcID: 412416.
I usually finish my story arcs As Soon As Possible when I start to outlevel them. That way they don't continue clutter the Clues tab while I move to the Next Big Thing.
Let's say an arc begins at level 1.
Some missions are intended for level 5.
I gain level 10, before I get to these missions. Now I've outgrown them.
Can I skip them and pick up the arc from a later mission or is it lost to me and I should Ouroboros it?