The objective of this study was to assess the possibility of retrofitting an existing full-scale wastewater treatment plant (WWTP) based on a sequencing batch reactor (SBR) technology with the enhanced biological phosphorus removal (EBPR) process. Wastewater characterisation showed highly variable influent composition that fluctuated throughout the year with a rather low and unstable SBOD/TP ratio (SBOD—soluble biological oxygen demand; TP—total phosphorus), which is considered unfavourable for EBPR. Characterisation of the sludge showed that the non-EBPR SBR sludge from the WWTP Koprivnica contained no detectable phosphorus accumulating organisms (PAO), but could be transformed in a laboratory into EBPR performing sludge in less than 45 days under favourable conditions for PAOs. The microbial community composition was assessed using an FISH (fluorescence in situ hybridization) analysis, which confirmed that the original sludge from the WWTP, which did not have detectable PAOs, was transformed into the sludge enriched by PAOs belonging to the genus ‘Candidatus Accumulibacter phosphatis’ after 43 days of cultivation. A plant retrofit, based on the results of laboratory experiments, was proposed with the enrichment of the wastewater with volatile fatty acids via primary anaerobic fermentation and step feeding. Results of mathematical modelling (BioWin) showed that such strategy could lead to sufficient P removal through EBPR in this WWTP.