they told me that if they make my upload better than it is, they will have to lower my downstream, is this correct?.
No; complete rubbish (but typical of the gibberish most 1st line support staff give out - basically either anything to make you go away, or just complete guesswork as they know little about the product - other than what's on their script).
ADSL2+ will support up to around 1.3Mbps upstream without having to lower the downstream. Annex M will boost the upstream further (providing your line is short enough), but this will reduce the downstream and I don't even know if Sky offer it in the first place.
My line is with BE (but also on ADSL2+).
My current upstream is 1.1Mbps, with a 36dB upstream line attenuation (58dB downstream), but to achieve this sync BE have me on a 3dB target margin (which applies to both up & downstream).
Your line will support the full 1.3Mbps (providing Sky offer it - a lot of providers around the World cap upstream sync speeds), but as Tommy45 has said, you may not do that with a 15dB target margin.
ADSL2+ with BE
DL Sync around 4.8Mbps
UL Sync 1088kbps
DG834GT with DGTeam firmware