I'm considering the possibility of moving the game I run over from 3rd to 4th edition. Unfortunately one (among many) of the issues that I have with 4th edition is the way that background count works. So I'm considering this variant. Background count applies only to drain codes for casting within them. i.e. Any spell cast within a BC will have its drain code raised by the BC, unless the BC is appropriately aspected in which case the drain code is reduced by the BC to a minimum of 1. Spells brought into the BC will have their force adjusted by the BC inversely. Adepts will lose effective magic for the purposes of calculating powers effects but not for how many powers they possess.
Does anyone foresee difficulties?

One other possibility I considered was raising the threshold for casting spells by the BC,However I considered this too harsh.

What's wrong with the RAW? What effect are you trying to get?

Avoiding recalculation of dice pools. Allowing high powered spells in the area whilst making sure there is a risk involved.

Your rule includes recalculation of dice pools: any adept power that involved a roll (Attribute Boost, for example) must take a penalty equal to the background count. Also, the RAW already adds the BC value to the Drain of all spells cast within it. Domains don't reduce Drain, but they add dice to all Magical Skill tests and Drain Resistance tests.

All in all, I can't see that your rule really changes much from RAW, other than being more lenient. If you want BC to not hurt as much, then good job. If you want it to hurt to the same degree, but in a different way, I don't think you've accomplished that.

That sounds fine for a houserule if you desire one. I, for one, do not see a problem with the way BC is written. It's supposed to be a game-changer.

Let's define the problem. What, specifically, do you not like about the RAW handling of background count?

A slightly off-topic question:

By "moving the game over" do you mean starting an all-new SR4 campaign with your existing SR group or transfering the current campaign into SR4?

If the latter I'd be curious how you plan to convert of the PCs. I've once thought about that myself and wonder how others would do that, since I didn't like the "official" conversion rules from the start of SR4. The "best" (for my taste) method I've come up with so far would be assigning stats that match your "mental image" of the character, calculate the cost according to Karmagen rules, then if needed, matching the Karma values of the different PCs.

I'm trying to keep the feel of how background counts have worked all along in our 3rd edition game, where target numbers and drain codes were affected but the players didn't suddenly have their maximum spells force reduced. I've clearly missed the bit in 4th ed about altering drain codes being too focused on the bit about dropping magic. By moving over I intend to have new characters in the same continuity with my players old PCs showing up now and again as contacts and occasionally Johnsons

Then allow me to propose an alternative: Nobody suffers a Magic penalty from background count, instead taking a dice pool penalty to all Magic-linked rolls. Keep everything else RAW: the increased drain, etc. I think this will give you the feel you're looking for, since the target number is fixed in SR4/4A. Bear in mind, though, that background count is, as Neraph pointed out, supposed to hit magical characters pretty hard. Unless you're comfortable with it being less onerous than in RAW, you may want to increase the penalty to (background count) x 2.

As I mentioned earlier I was considering adding a threshold instead of a dice pool modifier, perhaps 1 for each two points of BC (round up). Thoughts?

That's a pretty low threshold: even a Rating 6 background count (which by RAW reduces every non-initiate magician to effective-mundane status, and cripples virtually all initiates) would only impose a threshold of 3, which even a moderately optimized magician will beat with ease. Depending on how risky you want to make spellcasting, you could make mages resist drain regardless of whether they meet the threshold.

TBH, though, I think you should accept the RAW treatment of background counts as one of the changes to get used to in the new edition. You're going to be recalculating dice pools a lot anyway; that's just the way SR4/4A works. I think you'll find this is one area in which RAW is actually pretty good.

Honestly I think as is BC is done quite well, and is a fairly elegant system. With bonuses and penalties, you'll be recalculating dice pools pretty much all the time, and BC's don't introduce much more math. When your in a BC area, you lose dice on your test equal to the rating, and your force level before you start overcasting goes down by the rating, the math isn't that hard and it's a great and simple way to limit a magicians effectiveness.

I'd agree with Tanegar that increasing the threshold isn't a great idea. Most good magicians get most their dice from sources other than magic(skills, specialties, foci, ect), and it doesn't change the effect that high force spells would still have big effects. A BC 6 makes a rating 1 initiate is basically limited to palor tricks under the normal rules, but under your rules still means get can cast a force 7 mana bolt with little problem. Sure he'll get 3 less net hits, but the target wasn't going to resist it anyways.