If I replace the Lager yeast with an Ale yeast and ferment at Ale temps how much will my home brew be different? Will the beer be unrecognizable due to the change? Will it only have subtle differences?
It depends on the ale yeast - some are very clean, some produce lots of fruit flavours, some produce phenolic flavours and so on. It's a bit like asking if you can replace a female singer with a man. A drag artist may give a convincing "clone" performance, Pavarotti would give a performance that was amazing but completely different, but give me the mike and people would be running for the doors. We're all genetically different, and have been exposed to different environments - I will never have the innate talent of Pavarotti but I suspect with singing lessons I could at least hold a tune, conversely I probably have better genes for body shape than he did, but diet and exercise could have made him more slimline.
Although it's fashionable to talk about hops, yeast probably has more influence on the final taste of a beer - as well as contributing flavours of its own, it can change the sweetness and body of a beer, and even change the flavour profile of the hop compounds. Try a cheap and easy experiment - make 5gallons of a 5% SMaSH out of pale malt and Chinook and split it into 5 single gallons. Ferment them with different yeasts - if Fermentis are cheapest for you, then use US-05, BE-134, T-58, WB-06 (or Lallemand Munich Classic if you can get it) and your favourite lager yeast, or replace the lager yeast with S-33 if you're happy to just compare ale yeasts.
They're all ale yeasts but you will end up with very different beers - in particular if you compare the US-05 with the T-58 you'll notice that the T-58 is sweeter (less attenuation), the yeast contributes a slight pepperiness to the beer - and the grapefruit of the Chinook has turned into something more limey, you would think they were made with different hops. Such is the power of yeast. (You could do the same experiment with a more lager-y hop like Saaz, I don't know if T-58 will do such obvious things to Saaz though, quite possibly not)
Just generally - I wouldn't get too hung up on trying to define lager too exactly. It's one of those terms that the man on the street knows it when he see it, but biology doesn't like to be defined too precisely, the boundaries blur when you look at it in detail. For instance, you have California Commons (steam beers) that ferment lager yeast at ale temperatures, and kolsches that ferment ale yeasts at lager temperatures. So are they ales or lagers? As has been mentioned, it gets even messier once you start looking at DNA - WLP800 Pilsner is actually an ale yeast supposedly derived from a strain used to make Urquell, you can't get much more classic a lager than that! Conversely it turns out that WLP051 California V Ale is actually a lager yeast, but it was traditionally thought to be an ale yeast and was always fermented warm.
Also we now know that the lager family consists of two groups, Saaz and Frohberg, which are so different that it's been argued they should be considered different species. The Frohberg group (which includes 34/70) has more ale DNA, and can ferment cleanly at higher temperatures (see
this thread) than the Saaz group which contains the classic low-temperature lager yeasts.
Then again, you also have to consider fermentation conditions. Bud ferments their "lager" at 61F (16C), which they can get away with because the high pressures inside their huge fermenters suppresses the formation of off-flavours by the yeast. That's the sort of thing that's hard to replicate at a homebrew scale.
Bottom line - don't worry about the name, just concentrate on making the best liquid possible. A rose by any other name would smell as sweet and all that.
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