|Doug Miles Venture-Lieutenant, Michigan—Detroit|
So you are asking if it is OK to allow the party to play up to the 4-5 subtier despite the fact that their average level would not already allow them to do this? Or are they in between tiers and have the choice to play up or down?
Two issues here. Can they play up legally, and should they play up. Let's actually start with should they play up. You are correct that this adventure is pretty tame. The spot where I would feel leery is when the spriggans get large sized. They hit harder, have 10' reach and combat reflexes. It could go south quickly with a non-optimized party who get mixed up in melee. You know your players better than anyone, but personally I have a horrible track record when it comes to assessing the risk of playing up. "You guys should be fine" are the famous last words for many adventures where I encouraged the players to go for it.
I'm going to quote the Guide to PFS Organized Play just to make sure you understand the rules. "Determining Subtiers In order to determine which subtier a mixed-level group of PCs must play in, calculate the group’s average party level (APL). Divide the total number of character levels by the number of characters in the party. You should always round to the nearest whole number. If you are exactly at 0.5, let the group decide which subtier they wish to play.
"For scenarios written in Seasons 0 to 3, when the APL is in between subtiers, a party of six or seven characters must play the higher subtier. Parties with four or five characters must play the lower subtier. In the fringe case where there are no players that are high enough to have reached the subtier level (such as a party of six 3rd level characters), the group may decide to play down to the lower subtier."
Now can they play up despite the rules for Determining Subtiers explaining they cannot? I would say there's one exception. I would allow the party to play up under these conditions:
1) You get the appropriate gold & equipment access from the lower subtier. In other words, there is no greater reward for taking the risk at the higher tier. Whatever expendables they burn to defeat the high tier, that's the cost of taking on the challenge. This is like the Hard Mode in more recent scenarios.
2) Unanimous players decision. Everyone has to agree. If anyone feels this is a bad idea, it doesn't happen. They play the low tier.
3) No take-backs. If someone's PC dies as a result of the higher difficulty, they probably won't have the wealth at 2nd level to get Raised. Them's the breaks. You messed with the bull first, don't complain he has horns after you get gored.
I hope that helps you make a decision.