That really depends on where you are and who your service provider is.
For example, at the beginning of the 3G > 4G transition, 4G was made available to post-paid T-Mobile USA subscribers but not those on T-Mobile prepaid service nor MVNO customers using T-Mobile's towers.
The 4G > 5G transition was different and MVNO customers received access to T-Mobile 5G connectivity relatively quickly. I am using Mint Mobile -- a prepaid MVNO that accesses T-Mobile's towers. My handset (iPhone 12 mini) negotiates a 5G connection when the appropriate transmission equipment is in range. I certainly did NOT pay extra to get 5G.
At least for the Big 3 major cellular carriers here in the USA (Verizon, AT&T, T-Mobile/Sprint), the average consumer can't get a discount from their existing provider by selecting an older cellular technology.
Note that many of the improvements in 5G aren't about raw speed. There are more improvements in alleviating congestion (particularly in high-density venues like sports arenas, concert halls, public gathering areas, public transit hubs, etc.) as well as access to wireless frequencies that might have better performance in certain situations (like penetrating buildings).
That's why some 5G technologies such as mmWave have prioritized deployment in certain areas but other 5G technologies take precedence elsewhere.
The older cellular transmission technology is less cost effective and efficient than the newer technology. That's why cellular service providers are motivated to sunset old telecommunications standards and reprovision that frequency allocation to newer technology that can handle more simultaneous subscribers while providing a better product.
This is not unique to the cellular telecommunicatons industry. We saw this a while back in terrestrial television broadcast. The old analog standard was obsoleted, replaced by ATSC (in the USA) which allows for HD broadcast.
