Background: This is a comparative analysis of upgrade to cardiac resynchronisation therapy (CRT) of an existing pacemaker implanted for bradyarrhythmia or an implantable cardioverter defibrillator (ICD). International guidelines recommend an upgrade to CRT for both groups based on the same criteria; this may lead to a hypothesis of whether we are under-treating a population that would otherwise benefit more from a CRT than the other. Methods: So far, this is the largest multicentre retrospective study that includes in total, 151 (93 upgrades to CRT-P and 58 upgrades to CRT-D) participants who had an upgrade to a CRT device between January 2010 and January 2020. Clinical and echocardiographic parameters were analysed both, before and after an upgrade to CRT. Results: EF was greater in patients with PPM, both before and after upgrade. Moreover, the post upgrade increase in EF was significantly greater in patients with PPM. Patients with ICD have, on average, an NYHA class ranking greater than PPM patients, both before and after upgrade. Conclusions: These data show that the PPM group patients have a greater response to upgrade to CRT compared to those from the ICD group. It also demonstrates that patients with non-ischaemic cardiomyopathy are better responders compared to those with ischaemic aetiology.