It is necessary to also extend the upper control arms when extended lowers are installed. This is in order to maintain proper driveline angle. Not doing so will result in abnormal wear of the U-joint(s), vibration, and potential excess wear to the trans tailshaft bushing and can even lead to transmission failure.
Unfortunately, there are no "extended" upper control arms that are fixed length with the same characteristics as the OE stamped steel upper arms, which are designed to flex, and which seem to provide the best overall axle control without any real downside, as long as the bushings are in good condition. There is an exception, and that is cars that are lowered significantly. The flexibility of the OE arms masks the fact that at the extreme of travel the B-body rear suspension, as designed, is in a "bind" condition. Lowered cars operate with the suspension alot closer to the "edge" of rear suspension bind and often suffer problems with bushing wear, increased tendencty to incur wheel hop, harsher ride, etc.
The aftermarket does have adjustable rear upper arms. None of them have any flex designed in as do the stock upper arms, and many use polyurethane bushings, which have less compliance than the stock rubber, which was the OE choice for many years, though newer OE applications are now seeing use of more sophisticated materials, including urethanes.
Depending on the source, there are a number of available upper arms that use a spherical bearing at one end or at both ends instead of bushings. The use of bearings can eliminate most of the bind, however a price is usually paid--cost, bearing wear, driveline noise in the chassis/body/passenger compartment, and increased ride harshness are a few potential issues.
With adjustable arms, the biggest issue probably comes down to getting the ride height set where you intend to run it and then adjusting the arms to provide/restore the proper driveline angles.
A second issue is that WHEN the rear axle is placed in extended position, the mounting positions on the frame and rear axle housing for the upper control arm are no longer aligned properly with each other. This actually FORCES any regular rubber or poly bushing into static bind, with the car sitting still at normal ride height. This leads to problems typically seen when there are bushings at BOTH ends, such as torn bushings--a spherical bearing at one end does help but is not 100% effective at resolving this issue. I have not yet seen an adjustable upper arm that compensates for this misalignment--BUT I believe that something will eventually be produced that does.
There is an entire section in the Factory Service Manual devoted to driveline adjustment, and I will only suggest that anyone who plans to or has moved the rear axle become familiar with it.
Not to scare you away from the idea, but there are ancillary issues that go with moving the axle:
--exhaust system clearance, especially with aftermarket systems with larger pipe diameters
--change in relationship of upper/lower spring seats, thus rear spring angle/loading, much more critical with lowering springs
--change in shock absorber angle, and induced bind/wear on the mounts, which typically use a rubber or urethane bushing
--frame-to-axle brake hose length compensation--not usually a problem, but something to pay attention to
--rear axle pinion snubber contact location at top center of "nose" of differential, which may call for a longer bumper to contact sooner (especially if using a larger than stock driveshaft), or relocation of the bumper to avoid being chewed up by the pinion flange/deflector ring
--driveshaft slip yoke spline contact reduction which lowers overall torque capacity of the driveline (in reality it increases the area load on the slip yoke that is now LESS engaged with the trans output shaft = greater chance of twisting the splines). A longer slip yoke alone may or may not be sufficient to restore this to a "normal" or acceptable range, depending on the power level being applied. The act of moving the front U-joint away from its ideal location in relation to the tailshaft bushing of the transmission changes the loading that the transmission "sees" internally, and may eventually do harm. Bottom line, you have to consider a new driveshaft, regardless of what the extended arm supplier may tell you.