Can an antenna be "tuned" to different frequencies?
Sure... Assuming we're talking about a resonant design, you can tune it upwards in frequency with a pair of wire cutters or a hacksaw. No Prob.
Unfortunately the amateur community has taken to calling impedance matching devices "Antenna Tuners". With the exception of a remote matching device that connects directly to the antenna there are no magic boxes that can "tune" an antenna at the radio end of the coax. All they can do is match an impedance within their range to something that looks like 50 Ohms at the radio.
So... What's the difference? If an Antron 99 can be matched at 20 Meters it's matched, same as if it was really a 20 Meter antenna. Right?
The "tuner" will likely be operating on the ragged edge of its range and be lossy. Likewise the super low loss coax that's rated at 1 dB loss when terminated with its characteristic impedance will be seeing a high SWR and losses far greater than the spec sheet indicates. Then there's the antenna itself... Remember that resonance is a happy place where RF currents and voltages line up in phase for best efficiency. You won't be seeing that waaaay off resonance.
Combine the losses in the "tuner", feedline and the antenna when forcing a match off-resonance and there's nothing "tuned" about the system except for the match on the coax jumper between the radio and "tuner".
With something like the classic All-Band 136' Doublet fed with open wire parallel line a tuner is a reasonable compromise for the versatility it offers, but if you're thinking about a coax fed 6 Meter dipole "tuned" down to 10 Meters... That's a great way to turn a 100 watt rig into a QRP signal at the antenna.
If you can hang 66' of wire at a reasonable height, think about a short off center fed dipole (AKA "Windom") or G5RV instead. Reasonable performance on 40 through 10, they can load on Six, good SWL performance and not much of a fiscal commitment.http://www.packetradio.com/baseantennas.htm
Although I'm a fan of Six, starting out with a band that's open only slightly more often than Lehman Brothers is going to be frustrating...