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\subsection{Lepton Collider Upgrades to $\sqrt{s} = 500$~GeV}  Both the $\epem$ Higgs factories discussed in Section~\ref{sec:Higgs} have such high-energy upgrade projects. For the ILC, it is foreseen to double its length as to reach a centre-of-mass energy of 500 GeV. Is is also possible to increase to centre-of-mass energy of TLEP to $\sqrt{s} = 500$~GeV by tripling the RF length from 600 to 1700 m, and the total RF voltage from 12 to 35 GV, to compensate for the 31 GeV lost per turn by synchrotron radiation. The foreseen cost of this upgrade would amount to about two billion CHF. The one-minute beam lifetime achieved with a 2.5\% momentum acceptance at each interaction point would need to be compensated by an upgraded top-off injection, also needed for the TLEP luminosity upgrade, as alluded to in Section~\ref{sec:exp}. With these parameters, a instantaneous luminosity of $0.5\times10^{34}~\cms$ would be delivered at each interaction point, for a total instantaneous luminosity of $2 \times 10^{34}~\cms$ when summed over the four IPs, as displayed in Fig.~\ref{fig:TLEPUpgrade}. Although notbeing  included in the TLEP baseline programme at this time, the starting design study will investigate the feasibility of such an option and define the maximum reachable centre-of-mass energy within reasonable assumptions.