.... or tell her you need a new frame as the geometry of the old one is hurting your back, pretend to be changing components over in the garage (more free time to yourself) ....when all along you have simply replaced the entire bike and got the odd spanner out and squirted oil on your fingers whilst tinkering with your new toy.
Better still tell her you are merey spending money on spray paint as you fell off and damaged the paintwork on the frame ... then buy a can of spray paint and pretend you are creating a new look for your old bike but secretly throw it in a canal when she's not looking, or give it away to a friend several hundred miles away and unconnected to your woman's circle of friends (that'd possibly need to be on another planet as their underground networks are massive !) ... and then come in on the day of the new purchase looking slightly heady from the spray paint with the odd spot of paint on your hands and a few bits of bogus backing newspaper covered in the stuff grinning with satisfaction ... she'll think it is your ego puffed up from a good paint job and so you needn't hide the grin that the new dream machine has brought into your life !
.... or just buy new speakers for your stereo ..... and then try and convince yourself that you can feel the speed gain and smooth gear changing when you next get on your bike (requires almost the same leap of faith as simply fitting a new group set)
Serious response to your original question:-
If you change the cassette to a 10 speed you'll need a different chain size and the mech will need to move sufficiently to cover the 10 speed range (width of the cassette) or if the cassette is more compact you'll need to change the mech and the shifters as you'll need ones that move the cable and mech slightly less for each shift in gear .... appart from that you can mix and match the ratios to your hearts content on the cassette itself and for the chainset (within the range of what is available for the chainset .... with a small 30 tooth sprocket on the chainset at the front and 28 tooth selectable sprocket on the cassette at the back you have 30:28 which is approaching a 1:1 ratio for very very steep hills ... most riders will tell you this is uncool ... you may end up grinning however if you are able to climb to the top of a hill when they have had to dismount ... probably not necessary for winning triathlons though ?).
The ratios overlap substantially between front chainset and rear cassette combinations .... especially when the cassette increases to a 10spd .... the reason riders want 10 and 11 spd cassettes is more for the "tooth at a time" fine adjustment whilst riding on the same front sprocket, to help maintain even cadence and even effort as the road conditions, wind etc vary slightly and to help maintain even effort as you accelerate.
If you go for a triple chainset at the front you may need a new bottom bracket and front mech due to the extra width. This is only my understaning I have never done this myself! I am probably more of a novice cyclist than you are but this is my understanding and may help. Any good cycle shop will set you right though and shouldn'y let you buy components that don't marry up. I'd avoid e-bay for new components that i am fitting myself when I am still learning how things work and when the many many historical versions of the "same named" groups sets seem to get upgraded as often as Microsoft Windows! (i.e. Shimano 105's but which version, or what season etc). Good luck. S
My Specialized Allez Elite bought in 2005 has 105's on it as standard. Specialized have now downgraded the Allez standard groupset level to Tiagra for some strange reason .... I think the Tarmac range has taken over and is now given the higher range groupsets.
That said I imagine the newer Allez's with Tiagra groupsets may well be as good as the older 105 groupsets on the older Allez's .... as technology progresses, althought the base price of the new Allez is now somewhat lower than the old one used to be ... so perhaps not ?
I believe Specialized do however seem to create balanced bikes and as others have said upgrading a groupset may not be beneficial as you may pay almost the cost of a bike to do so and then the wheels will be substandard for the other componentry you currently have etc etc .... bit like buying better speakers for a stereo only to find they show up the substandard quality of the amp ... or perhaps worse buying a better amp and hearing no difference due to substandard speakers and not realising that anything has changed at all except your bank balance (not sure which is the worst way around to be honest ?)
Why not shop for a good secondhand bike instead with better spec? ... somebody mught be flogging an old Colnago with Campy shifters and Ultegra mechs, chainsets and clusters ... then your laughing as you cash in on their old gear just because of their latest desire for the S-Works Tarmac SL3 simply because Contador and Schleck happened to both be riding one this year !!! .... you might pick something special up for the price of a groupset ... an if you so love the geometry of your existing frame and like building bikes you can migrate the whole lot onto your old frame, wheels an all !
Not sure why I suddenly posted to this old thread ... anyway have fun. S