No, your older TV will still receive the pictures and audio for all channels from a Freeview HD decoder, but they will be in Standard Definition (SD). Chances are they’ll still look a lot sharper than the analogue pictures you’ve been used to, though they’ll pale in comparison to HD pictures. All Freeview HD decoders are equipped with connections that can be used with your older TVs, though most don’t have RF outputs (the type of socket used by your TV for the aerial), so if your TV is really old and doesn’t have any AV inputs (coloured RCA sockets), you may need to purchase an additional modulator (converter) or use connect via a VCR or similar that has a modulator built in as part of the tuner system.
The Freeview approved decoders will output HD only from an HDMI socket, but other non-approved decoders may also output HD on component. Component is capable of HD pictures up to 1080i (which is as high as Freeview goes), the RCA sockets on the TV are often labelled Y, Pb and Pr, and are coloured red, green and blue. Audio is transmitted separately via two RCA sockets coloured red and white (or occasionally black). Note that not all component inputs on TVs are created equal, more often than not, they are not capable of HD pictures, but check the manual as you may have one of the ones that does.
When the time comes and you do upgrade your TV to an HD capable TV, it’s simply a case of plugging in an HDMI cable (usually supplied with the decoder), and you’ll be up and running in High Definition.